django/tests/backends/tests.py

# -*- coding: utf-8 -*-
# Unit and doctests for specific database backends.
from __future__ import absolute_import, unicode_literals

import datetime
from decimal import Decimal
import threading

from django.conf import settings
from django.core.management.color import no_style
from django.db import (connection, connections, DEFAULT_DB_ALIAS,
    DatabaseError, IntegrityError, transaction)
from django.db.backends.signals import connection_created
from django.db.backends.postgresql_psycopg2 import version as pg_version
from django.db.backends.util import format_number
from django.db.models import Sum, Avg, Variance, StdDev
from django.db.utils import ConnectionHandler
from django.test import (TestCase, skipUnlessDBFeature, skipIfDBFeature,
    TransactionTestCase)
from django.test.utils import override_settings, str_prefix
from django.utils import six, unittest
from django.utils.six.moves import xrange

from . import models


class DummyBackendTest(TestCase):
    def test_no_databases(self):
        """
        Test that empty DATABASES setting default to the dummy backend.
        """
        DATABASES = {}
        conns = ConnectionHandler(DATABASES)
        self.assertEqual(conns[DEFAULT_DB_ALIAS].settings_dict['ENGINE'],
            'django.db.backends.dummy')


class OracleChecks(unittest.TestCase):

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle quote_name semantics")
    def test_quote_name(self):
        # Check that '%' chars are escaped for query execution.
        name = '"SOME%NAME"'
        quoted_name = connection.ops.quote_name(name)
        self.assertEqual(quoted_name % (), name)

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle cursor semantics")
    def test_dbms_session(self):
        # If the backend is Oracle, test that we can call a standard
        # stored procedure through our cursor wrapper.
        from django.db.backends.oracle.base import convert_unicode

        cursor = connection.cursor()
        cursor.callproc(convert_unicode('DBMS_SESSION.SET_IDENTIFIER'),
                        [convert_unicode('_django_testing!')])

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle cursor semantics")
    def test_cursor_var(self):
        # If the backend is Oracle, test that we can pass cursor variables
        # as query parameters.
        from django.db.backends.oracle.base import Database

        cursor = connection.cursor()
        var = cursor.var(Database.STRING)
        cursor.execute("BEGIN %s := 'X'; END; ", [var])
        self.assertEqual(var.getvalue(), 'X')

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle cursor semantics")
    def test_long_string(self):
        # If the backend is Oracle, test that we can save a text longer
        # than 4000 chars and read it properly
        c = connection.cursor()
        c.execute('CREATE TABLE ltext ("TEXT" NCLOB)')
        long_str = ''.join([six.text_type(x) for x in xrange(4000)])
        c.execute('INSERT INTO ltext VALUES (%s)', [long_str])
        c.execute('SELECT text FROM ltext')
        row = c.fetchone()
        self.assertEqual(long_str, row[0].read())
        c.execute('DROP TABLE ltext')

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle connection semantics")
    def test_client_encoding(self):
        # If the backend is Oracle, test that the client encoding is set
        # correctly.  This was broken under Cygwin prior to r14781.
        connection.cursor()  # Ensure the connection is initialized.
        self.assertEqual(connection.connection.encoding, "UTF-8")
        self.assertEqual(connection.connection.nencoding, "UTF-8")

    @unittest.skipUnless(connection.vendor == 'oracle',
                         "No need to check Oracle connection semantics")
    def test_order_of_nls_parameters(self):
        # an 'almost right' datetime should work with configured
        # NLS parameters as per #18465.
        c = connection.cursor()
        query = "select 1 from dual where '1936-12-29 00:00' < sysdate"
        # Test that the query succeeds without errors - pre #18465 this
        # wasn't the case.
        c.execute(query)
        self.assertEqual(c.fetchone()[0], 1)


class MySQLTests(TestCase):
    @unittest.skipUnless(connection.vendor == 'mysql',
                        "Test valid only for MySQL")
    def test_autoincrement(self):
        """
        Check that auto_increment fields are reset correctly by sql_flush().
        Before MySQL version 5.0.13 TRUNCATE did not do auto_increment reset.
        Refs #16961.
        """
        statements = connection.ops.sql_flush(no_style(),
                                              tables=['test'],
                                              sequences=[{
                                                  'table': 'test',
                                                  'col': 'somecol',
                                              }])
        found_reset = False
        for sql in statements:
            found_reset = found_reset or 'ALTER TABLE' in sql
        if connection.mysql_version < (5, 0, 13):
            self.assertTrue(found_reset)
        else:
            self.assertFalse(found_reset)


class DateQuotingTest(TestCase):

    def test_django_date_trunc(self):
        """
        Test the custom ``django_date_trunc method``, in particular against
        fields which clash with strings passed to it (e.g. 'year') - see
        #12818__.

        __: http://code.djangoproject.com/ticket/12818

        """
        updated = datetime.datetime(2010, 2, 20)
        models.SchoolClass.objects.create(year=2009, last_updated=updated)
        years = models.SchoolClass.objects.dates('last_updated', 'year')
        self.assertEqual(list(years), [datetime.date(2010, 1, 1)])

    def test_django_date_extract(self):
        """
        Test the custom ``django_date_extract method``, in particular against fields
        which clash with strings passed to it (e.g. 'day') - see #12818__.

        __: http://code.djangoproject.com/ticket/12818

        """
        updated = datetime.datetime(2010, 2, 20)
        models.SchoolClass.objects.create(year=2009, last_updated=updated)
        classes = models.SchoolClass.objects.filter(last_updated__day=20)
        self.assertEqual(len(classes), 1)


@override_settings(DEBUG=True)
class LastExecutedQueryTest(TestCase):

    def test_debug_sql(self):
        list(models.Reporter.objects.filter(first_name="test"))
        sql = connection.queries[-1]['sql'].lower()
        self.assertIn("select", sql)
        self.assertIn(models.Reporter._meta.db_table, sql)

    def test_query_encoding(self):
        """
        Test that last_executed_query() returns an Unicode string
        """
        persons = models.Reporter.objects.filter(raw_data=b'\x00\x46  \xFE').extra(select={'föö': 1})
        sql, params = persons.query.sql_with_params()
        cursor = persons.query.get_compiler('default').execute_sql(None)
        last_sql = cursor.db.ops.last_executed_query(cursor, sql, params)
        self.assertIsInstance(last_sql, six.text_type)

    @unittest.skipUnless(connection.vendor == 'sqlite',
                         "This test is specific to SQLite.")
    def test_no_interpolation_on_sqlite(self):
        # Regression for #17158
        # This shouldn't raise an exception
        query = "SELECT strftime('%Y', 'now');"
        connection.cursor().execute(query)
        self.assertEqual(connection.queries[-1]['sql'],
            str_prefix("QUERY = %(_)s\"SELECT strftime('%%Y', 'now');\" - PARAMS = ()"))


class ParameterHandlingTest(TestCase):
    def test_bad_parameter_count(self):
        "An executemany call with too many/not enough parameters will raise an exception (Refs #12612)"
        cursor = connection.cursor()
        query = ('INSERT INTO %s (%s, %s) VALUES (%%s, %%s)' % (
            connection.introspection.table_name_converter('backends_square'),
            connection.ops.quote_name('root'),
            connection.ops.quote_name('square')
        ))
        self.assertRaises(Exception, cursor.executemany, query, [(1, 2, 3)])
        self.assertRaises(Exception, cursor.executemany, query, [(1,)])


# Unfortunately, the following tests would be a good test to run on all
# backends, but it breaks MySQL hard. Until #13711 is fixed, it can't be run
# everywhere (although it would be an effective test of #13711).
class LongNameTest(TestCase):
    """Long primary keys and model names can result in a sequence name
    that exceeds the database limits, which will result in truncation
    on certain databases (e.g., Postgres). The backend needs to use
    the correct sequence name in last_insert_id and other places, so
    check it is. Refs #8901.
    """

    @skipUnlessDBFeature('supports_long_model_names')
    def test_sequence_name_length_limits_create(self):
        """Test creation of model with long name and long pk name doesn't error. Ref #8901"""
        models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()

    @skipUnlessDBFeature('supports_long_model_names')
    def test_sequence_name_length_limits_m2m(self):
        """Test an m2m save of a model with a long name and a long m2m field name doesn't error as on Django >=1.2 this now uses object saves. Ref #8901"""
        obj = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()
        rel_obj = models.Person.objects.create(first_name='Django', last_name='Reinhardt')
        obj.m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.add(rel_obj)

    @skipUnlessDBFeature('supports_long_model_names')
    def test_sequence_name_length_limits_flush(self):
        """Test that sequence resetting as part of a flush with model with long name and long pk name doesn't error. Ref #8901"""
        # A full flush is expensive to the full test, so we dig into the
        # internals to generate the likely offending SQL and run it manually

        # Some convenience aliases
        VLM = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
        VLM_m2m = VLM.m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.through
        tables = [
            VLM._meta.db_table,
            VLM_m2m._meta.db_table,
        ]
        sequences = [
            {
                'column': VLM._meta.pk.column,
                'table': VLM._meta.db_table
            },
        ]
        cursor = connection.cursor()
        for statement in connection.ops.sql_flush(no_style(), tables, sequences):
            cursor.execute(statement)


class SequenceResetTest(TestCase):
    def test_generic_relation(self):
        "Sequence names are correct when resetting generic relations (Ref #13941)"
        # Create an object with a manually specified PK
        models.Post.objects.create(id=10, name='1st post', text='hello world')

        # Reset the sequences for the database
        cursor = connection.cursor()
        commands = connections[DEFAULT_DB_ALIAS].ops.sequence_reset_sql(no_style(), [models.Post])
        for sql in commands:
            cursor.execute(sql)

        # If we create a new object now, it should have a PK greater
        # than the PK we specified manually.
        obj = models.Post.objects.create(name='New post', text='goodbye world')
        self.assertTrue(obj.pk > 10)


class PostgresVersionTest(TestCase):
    def assert_parses(self, version_string, version):
        self.assertEqual(pg_version._parse_version(version_string), version)

    def test_parsing(self):
        """Test PostgreSQL version parsing from `SELECT version()` output"""
        self.assert_parses("PostgreSQL 8.3 beta4", 80300)
        self.assert_parses("PostgreSQL 8.3", 80300)
        self.assert_parses("EnterpriseDB 8.3", 80300)
        self.assert_parses("PostgreSQL 8.3.6", 80306)
        self.assert_parses("PostgreSQL 8.4beta1", 80400)
        self.assert_parses("PostgreSQL 8.3.1 on i386-apple-darwin9.2.2, compiled by GCC i686-apple-darwin9-gcc-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5478)", 80301)

    def test_version_detection(self):
        """Test PostgreSQL version detection"""

        # Helper mocks
        class CursorMock(object):
            "Very simple mock of DB-API cursor"
            def execute(self, arg):
                pass

            def fetchone(self):
                return ["PostgreSQL 8.3"]

        class OlderConnectionMock(object):
            "Mock of psycopg2 (< 2.0.12) connection"
            def cursor(self):
                return CursorMock()

        # psycopg2 < 2.0.12 code path
        conn = OlderConnectionMock()
        self.assertEqual(pg_version.get_version(conn), 80300)


class PostgresNewConnectionTest(TestCase):
    """
    #17062: PostgreSQL shouldn't roll back SET TIME ZONE, even if the first
    transaction is rolled back.
    """
    @unittest.skipUnless(
        connection.vendor == 'postgresql',
        "This test applies only to PostgreSQL")
    def test_connect_and_rollback(self):
        new_connections = ConnectionHandler(settings.DATABASES)
        new_connection = new_connections[DEFAULT_DB_ALIAS]
        try:
            # Ensure the database default time zone is different than
            # the time zone in new_connection.settings_dict. We can
            # get the default time zone by reset & show.
            cursor = new_connection.cursor()
            cursor.execute("RESET TIMEZONE")
            cursor.execute("SHOW TIMEZONE")
            db_default_tz = cursor.fetchone()[0]
            new_tz = 'Europe/Paris' if db_default_tz == 'UTC' else 'UTC'
            new_connection.close()

            # Fetch a new connection with the new_tz as default
            # time zone, run a query and rollback.
            new_connection.settings_dict['TIME_ZONE'] = new_tz
            new_connection.enter_transaction_management()
            cursor = new_connection.cursor()
            new_connection.rollback()

            # Now let's see if the rollback rolled back the SET TIME ZONE.
            cursor.execute("SHOW TIMEZONE")
            tz = cursor.fetchone()[0]
            self.assertEqual(new_tz, tz)
        finally:
            try:
                new_connection.close()
            except DatabaseError:
                pass


# This test needs to run outside of a transaction, otherwise closing the
# connection would implicitly rollback and cause problems during teardown.
class ConnectionCreatedSignalTest(TransactionTestCase):

    # Unfortunately with sqlite3 the in-memory test database cannot be closed,
    # and so it cannot be re-opened during testing.
    @skipUnlessDBFeature('test_db_allows_multiple_connections')
    def test_signal(self):
        data = {}

        def receiver(sender, connection, **kwargs):
            data["connection"] = connection

        connection_created.connect(receiver)
        connection.close()
        connection.cursor()
        self.assertTrue(data["connection"].connection is connection.connection)

        connection_created.disconnect(receiver)
        data.clear()
        connection.cursor()
        self.assertTrue(data == {})


class EscapingChecks(TestCase):
    """
    All tests in this test case are also run with settings.DEBUG=True in
    EscapingChecksDebug test case, to also test CursorDebugWrapper.
    """

    # For Oracle, when you want to select a value, you need to specify the
    # special pseudo-table 'dual'; a select with no from clause is invalid.
    bare_select_suffix = " FROM DUAL" if connection.vendor == 'oracle' else ""

    def test_paramless_no_escaping(self):
        cursor = connection.cursor()
        cursor.execute("SELECT '%s'" + self.bare_select_suffix)
        self.assertEqual(cursor.fetchall()[0][0], '%s')

    def test_parameter_escaping(self):
        cursor = connection.cursor()
        cursor.execute("SELECT '%%', %s" + self.bare_select_suffix, ('%d',))
        self.assertEqual(cursor.fetchall()[0], ('%', '%d'))

    @unittest.skipUnless(connection.vendor == 'sqlite',
                         "This is a sqlite-specific issue")
    def test_sqlite_parameter_escaping(self):
        #13648: '%s' escaping support for sqlite3
        cursor = connection.cursor()
        cursor.execute("select strftime('%s', date('now'))")
        response = cursor.fetchall()[0][0]
        # response should be an non-zero integer
        self.assertTrue(int(response))

@override_settings(DEBUG=True)
class EscapingChecksDebug(EscapingChecks):
    pass


class SqlliteAggregationTests(TestCase):
    """
    #19360: Raise NotImplementedError when aggregating on date/time fields.
    """
    @unittest.skipUnless(connection.vendor == 'sqlite',
                         "No need to check SQLite aggregation semantics")
    def test_aggregation(self):
        for aggregate in (Sum, Avg, Variance, StdDev):
            self.assertRaises(NotImplementedError,
                models.Item.objects.all().aggregate, aggregate('time'))
            self.assertRaises(NotImplementedError,
                models.Item.objects.all().aggregate, aggregate('date'))
            self.assertRaises(NotImplementedError,
                models.Item.objects.all().aggregate, aggregate('last_modified'))


class BackendTestCase(TestCase):

    def create_squares_with_executemany(self, args):
        cursor = connection.cursor()
        opts = models.Square._meta
        tbl = connection.introspection.table_name_converter(opts.db_table)
        f1 = connection.ops.quote_name(opts.get_field('root').column)
        f2 = connection.ops.quote_name(opts.get_field('square').column)
        query = 'INSERT INTO %s (%s, %s) VALUES (%%s, %%s)' % (tbl, f1, f2)
        cursor.executemany(query, args)

    def test_cursor_executemany(self):
        #4896: Test cursor.executemany
        args = [(i, i**2) for i in range(-5, 6)]
        self.create_squares_with_executemany(args)
        self.assertEqual(models.Square.objects.count(), 11)
        for i in range(-5, 6):
            square = models.Square.objects.get(root=i)
            self.assertEqual(square.square, i**2)

    def test_cursor_executemany_with_empty_params_list(self):
        #4765: executemany with params=[] does nothing
        args = []
        self.create_squares_with_executemany(args)
        self.assertEqual(models.Square.objects.count(), 0)

    def test_cursor_executemany_with_iterator(self):
        #10320: executemany accepts iterators
        args = iter((i, i**2) for i in range(-3, 2))
        self.create_squares_with_executemany(args)
        self.assertEqual(models.Square.objects.count(), 5)

        args = iter((i, i**2) for i in range(3, 7))
        with override_settings(DEBUG=True):
            # same test for DebugCursorWrapper
            self.create_squares_with_executemany(args)
        self.assertEqual(models.Square.objects.count(), 9)

    def test_unicode_fetches(self):
        #6254: fetchone, fetchmany, fetchall return strings as unicode objects
        qn = connection.ops.quote_name
        models.Person(first_name="John", last_name="Doe").save()
        models.Person(first_name="Jane", last_name="Doe").save()
        models.Person(first_name="Mary", last_name="Agnelline").save()
        models.Person(first_name="Peter", last_name="Parker").save()
        models.Person(first_name="Clark", last_name="Kent").save()
        opts2 = models.Person._meta
        f3, f4 = opts2.get_field('first_name'), opts2.get_field('last_name')
        query2 = ('SELECT %s, %s FROM %s ORDER BY %s'
          % (qn(f3.column), qn(f4.column), connection.introspection.table_name_converter(opts2.db_table),
             qn(f3.column)))
        cursor = connection.cursor()
        cursor.execute(query2)
        self.assertEqual(cursor.fetchone(), ('Clark', 'Kent'))
        self.assertEqual(list(cursor.fetchmany(2)), [('Jane', 'Doe'), ('John', 'Doe')])
        self.assertEqual(list(cursor.fetchall()), [('Mary', 'Agnelline'), ('Peter', 'Parker')])

    def test_unicode_password(self):
        old_password = connection.settings_dict['PASSWORD']
        connection.settings_dict['PASSWORD'] = "françois"
        try:
            connection.cursor()
        except DatabaseError:
            # As password is probably wrong, a database exception is expected
            pass
        except Exception as e:
            self.fail("Unexpected error raised with unicode password: %s" % e)
        finally:
            connection.settings_dict['PASSWORD'] = old_password

    def test_database_operations_helper_class(self):
        # Ticket #13630
        self.assertTrue(hasattr(connection, 'ops'))
        self.assertTrue(hasattr(connection.ops, 'connection'))
        self.assertEqual(connection, connection.ops.connection)

    def test_cached_db_features(self):
        self.assertIn(connection.features.supports_transactions, (True, False))
        self.assertIn(connection.features.supports_stddev, (True, False))
        self.assertIn(connection.features.can_introspect_foreign_keys, (True, False))

    def test_duplicate_table_error(self):
        """ Test that creating an existing table returns a DatabaseError """
        cursor = connection.cursor()
        query = 'CREATE TABLE %s (id INTEGER);' % models.Article._meta.db_table
        with self.assertRaises(DatabaseError):
            cursor.execute(query)


# We don't make these tests conditional because that means we would need to
# check and differentiate between:
# * MySQL+InnoDB, MySQL+MYISAM (something we currently can't do).
# * if sqlite3 (if/once we get #14204 fixed) has referential integrity turned
#   on or not, something that would be controlled by runtime support and user
#   preference.
# verify if its type is django.database.db.IntegrityError.
class FkConstraintsTests(TransactionTestCase):

    def setUp(self):
        # Create a Reporter.
        self.r = models.Reporter.objects.create(first_name='John', last_name='Smith')

    def test_integrity_checks_on_creation(self):
        """
        Try to create a model instance that violates a FK constraint. If it
        fails it should fail with IntegrityError.
        """
        a = models.Article(headline="This is a test", pub_date=datetime.datetime(2005, 7, 27), reporter_id=30)
        try:
            a.save()
        except IntegrityError:
            return
        self.skipTest("This backend does not support integrity checks.")

    def test_integrity_checks_on_update(self):
        """
        Try to update a model instance introducing a FK constraint violation.
        If it fails it should fail with IntegrityError.
        """
        # Create an Article.
        models.Article.objects.create(headline="Test article", pub_date=datetime.datetime(2010, 9, 4), reporter=self.r)
        # Retrive it from the DB
        a = models.Article.objects.get(headline="Test article")
        a.reporter_id = 30
        try:
            a.save()
        except IntegrityError:
            return
        self.skipTest("This backend does not support integrity checks.")

    def test_disable_constraint_checks_manually(self):
        """
        When constraint checks are disabled, should be able to write bad data without IntegrityErrors.
        """
        transaction.set_autocommit(False)
        try:
            # Create an Article.
            models.Article.objects.create(headline="Test article", pub_date=datetime.datetime(2010, 9, 4), reporter=self.r)
            # Retrive it from the DB
            a = models.Article.objects.get(headline="Test article")
            a.reporter_id = 30
            try:
                connection.disable_constraint_checking()
                a.save()
                connection.enable_constraint_checking()
            except IntegrityError:
                self.fail("IntegrityError should not have occurred.")
            finally:
                transaction.rollback()
        finally:
            transaction.set_autocommit(True)

    def test_disable_constraint_checks_context_manager(self):
        """
        When constraint checks are disabled (using context manager), should be able to write bad data without IntegrityErrors.
        """
        transaction.set_autocommit(False)
        try:
            # Create an Article.
            models.Article.objects.create(headline="Test article", pub_date=datetime.datetime(2010, 9, 4), reporter=self.r)
            # Retrive it from the DB
            a = models.Article.objects.get(headline="Test article")
            a.reporter_id = 30
            try:
                with connection.constraint_checks_disabled():
                    a.save()
            except IntegrityError:
                self.fail("IntegrityError should not have occurred.")
            finally:
                transaction.rollback()
        finally:
            transaction.set_autocommit(True)

    def test_check_constraints(self):
        """
        Constraint checks should raise an IntegrityError when bad data is in the DB.
        """
        try:
            transaction.set_autocommit(False)
            # Create an Article.
            models.Article.objects.create(headline="Test article", pub_date=datetime.datetime(2010, 9, 4), reporter=self.r)
            # Retrive it from the DB
            a = models.Article.objects.get(headline="Test article")
            a.reporter_id = 30
            try:
                with connection.constraint_checks_disabled():
                    a.save()
                    with self.assertRaises(IntegrityError):
                        connection.check_constraints()
            finally:
                transaction.rollback()
        finally:
            transaction.set_autocommit(True)


class ThreadTests(TestCase):

    def test_default_connection_thread_local(self):
        """
        Ensure that the default connection (i.e. django.db.connection) is
        different for each thread.
        Refs #17258.
        """
        # Map connections by id because connections with identical aliases
        # have the same hash.
        connections_dict = {}
        connection.cursor()
        connections_dict[id(connection)] = connection

        def runner():
            # Passing django.db.connection between threads doesn't work while
            # connections[DEFAULT_DB_ALIAS] does.
            from django.db import connections
            connection = connections[DEFAULT_DB_ALIAS]
            # Allow thread sharing so the connection can be closed by the
            # main thread.
            connection.allow_thread_sharing = True
            connection.cursor()
            connections_dict[id(connection)] = connection
        for x in range(2):
            t = threading.Thread(target=runner)
            t.start()
            t.join()
        # Check that each created connection got different inner connection.
        self.assertEqual(
            len(set(conn.connection for conn in connections_dict.values())),
            3)
        # Finish by closing the connections opened by the other threads (the
        # connection opened in the main thread will automatically be closed on
        # teardown).
        for conn in connections_dict.values():
            if conn is not connection:
                conn.close()

    def test_connections_thread_local(self):
        """
        Ensure that the connections are different for each thread.
        Refs #17258.
        """
        # Map connections by id because connections with identical aliases
        # have the same hash.
        connections_dict = {}
        for conn in connections.all():
            connections_dict[id(conn)] = conn

        def runner():
            from django.db import connections
            for conn in connections.all():
                # Allow thread sharing so the connection can be closed by the
                # main thread.
                conn.allow_thread_sharing = True
                connections_dict[id(conn)] = conn
        for x in range(2):
            t = threading.Thread(target=runner)
            t.start()
            t.join()
        self.assertEqual(len(connections_dict), 6)
        # Finish by closing the connections opened by the other threads (the
        # connection opened in the main thread will automatically be closed on
        # teardown).
        for conn in connections_dict.values():
            if conn is not connection:
                conn.close()

    def test_pass_connection_between_threads(self):
        """
        Ensure that a connection can be passed from one thread to the other.
        Refs #17258.
        """
        models.Person.objects.create(first_name="John", last_name="Doe")

        def do_thread():
            def runner(main_thread_connection):
                from django.db import connections
                connections['default'] = main_thread_connection
                try:
                    models.Person.objects.get(first_name="John", last_name="Doe")
                except Exception as e:
                    exceptions.append(e)
            t = threading.Thread(target=runner, args=[connections['default']])
            t.start()
            t.join()

        # Without touching allow_thread_sharing, which should be False by default.
        exceptions = []
        do_thread()
        # Forbidden!
        self.assertIsInstance(exceptions[0], DatabaseError)

        # If explicitly setting allow_thread_sharing to False
        connections['default'].allow_thread_sharing = False
        exceptions = []
        do_thread()
        # Forbidden!
        self.assertIsInstance(exceptions[0], DatabaseError)

        # If explicitly setting allow_thread_sharing to True
        connections['default'].allow_thread_sharing = True
        exceptions = []
        do_thread()
        # All good
        self.assertEqual(exceptions, [])

    def test_closing_non_shared_connections(self):
        """
        Ensure that a connection that is not explicitly shareable cannot be
        closed by another thread.
        Refs #17258.
        """
        # First, without explicitly enabling the connection for sharing.
        exceptions = set()

        def runner1():
            def runner2(other_thread_connection):
                try:
                    other_thread_connection.close()
                except DatabaseError as e:
                    exceptions.add(e)
            t2 = threading.Thread(target=runner2, args=[connections['default']])
            t2.start()
            t2.join()
        t1 = threading.Thread(target=runner1)
        t1.start()
        t1.join()
        # The exception was raised
        self.assertEqual(len(exceptions), 1)

        # Then, with explicitly enabling the connection for sharing.
        exceptions = set()

        def runner1():
            def runner2(other_thread_connection):
                try:
                    other_thread_connection.close()
                except DatabaseError as e:
                    exceptions.add(e)
            # Enable thread sharing
            connections['default'].allow_thread_sharing = True
            t2 = threading.Thread(target=runner2, args=[connections['default']])
            t2.start()
            t2.join()
        t1 = threading.Thread(target=runner1)
        t1.start()
        t1.join()
        # No exception was raised
        self.assertEqual(len(exceptions), 0)


class MySQLPKZeroTests(TestCase):
    """
    Zero as id for AutoField should raise exception in MySQL, because MySQL
    does not allow zero for automatic primary key.
    """

    @skipIfDBFeature('allows_primary_key_0')
    def test_zero_as_autoval(self):
        with self.assertRaises(ValueError):
            models.Square.objects.create(id=0, root=0, square=1)


class DBConstraintTestCase(TransactionTestCase):
    def test_can_reference_existant(self):
        obj = models.Object.objects.create()
        ref = models.ObjectReference.objects.create(obj=obj)
        self.assertEqual(ref.obj, obj)

        ref = models.ObjectReference.objects.get(obj=obj)
        self.assertEqual(ref.obj, obj)

    def test_can_reference_non_existant(self):
        self.assertFalse(models.Object.objects.filter(id=12345).exists())
        ref = models.ObjectReference.objects.create(obj_id=12345)
        ref_new = models.ObjectReference.objects.get(obj_id=12345)
        self.assertEqual(ref, ref_new)

        with self.assertRaises(models.Object.DoesNotExist):
            ref.obj

    def test_many_to_many(self):
        obj = models.Object.objects.create()
        obj.related_objects.create()
        self.assertEqual(models.Object.objects.count(), 2)
        self.assertEqual(obj.related_objects.count(), 1)

        intermediary_model = models.Object._meta.get_field_by_name("related_objects")[0].rel.through
        intermediary_model.objects.create(from_object_id=obj.id, to_object_id=12345)
        self.assertEqual(obj.related_objects.count(), 1)
        self.assertEqual(intermediary_model.objects.count(), 2)


class BackendUtilTests(TestCase):

    def test_format_number(self):
        """
        Test the format_number converter utility
        """
        def equal(value, max_d, places, result):
            self.assertEqual(format_number(Decimal(value), max_d, places), result)

        equal('0', 12, 3,
              '0.000')
        equal('0', 12, 8,
              '0.00000000')
        equal('1', 12, 9,
              '1.000000000')
        equal('0.00000000', 12, 8,
              '0.00000000')
        equal('0.000000004', 12, 8,
              '0.00000000')
        equal('0.000000008', 12, 8,
              '0.00000001')
        equal('0.000000000000000000999', 10, 8,
              '0.00000000')
        equal('0.1234567890', 12, 10,
              '0.1234567890')
        equal('0.1234567890', 12, 9,
              '0.123456789')
        equal('0.1234567890', 12, 8,
              '0.12345679')
        equal('0.1234567890', 12, 5,
              '0.12346')
        equal('0.1234567890', 12, 3,
              '0.123')
        equal('0.1234567890', 12, 1,
              '0.1')
        equal('0.1234567890', 12, 0,
              '0')