Added a "depth" argument to select_related() to control how many "levels" of relations select_related() is willing to follow (refs #3275).

Also added unit tests for select_related().

git-svn-id: http://code.djangoproject.com/svn/django/trunk@4645 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -84,6 +84,7 @@ class QuerySet(object):
         self._filters = Q()
         self._order_by = None        # Ordering, e.g. ('date', '-name'). If None, use model's ordering.
         self._select_related = False # Whether to fill cache for related objects.
+        self._max_related_depth = 0  # Maximum "depth" for select_related
         self._distinct = False       # Whether the query should use SELECT DISTINCT.
         self._select = {}            # Dictionary of attname -> SQL.
         self._where = []             # List of extra WHERE clauses to use.
@@ -186,7 +187,8 @@ class QuerySet(object):
                 raise StopIteration
             for row in rows:
                 if fill_cache:
-                    obj, index_end = get_cached_row(self.model, row, 0)
+                    obj, index_end = get_cached_row(klass=self.model, row=row, 
+                                                    index_start=0, max_depth=self._max_related_depth)
                 else:
                     obj = self.model(*row[:index_end])
                 for i, k in enumerate(extra_select):
@@ -394,9 +396,9 @@ class QuerySet(object):
         else:
             return self._filter_or_exclude(None, **filter_obj)
 
-    def select_related(self, true_or_false=True):
+    def select_related(self, true_or_false=True, depth=0):
         "Returns a new QuerySet instance with '_select_related' modified."
-        return self._clone(_select_related=true_or_false)
+        return self._clone(_select_related=true_or_false, _max_related_depth=depth)
 
     def order_by(self, *field_names):
         "Returns a new QuerySet instance with the ordering changed."
@@ -430,6 +432,7 @@ class QuerySet(object):
         c._filters = self._filters
         c._order_by = self._order_by
         c._select_related = self._select_related
+        c._max_related_depth = self._max_related_depth
         c._distinct = self._distinct
         c._select = self._select.copy()
         c._where = self._where[:]
@@ -483,7 +486,10 @@ class QuerySet(object):
 
         # Add additional tables and WHERE clauses based on select_related.
         if self._select_related:
-            fill_table_cache(opts, select, tables, where, opts.db_table, [opts.db_table])
+            fill_table_cache(opts, select, tables, where, 
+                             old_prefix=opts.db_table, 
+                             cache_tables_seen=[opts.db_table], 
+                             max_depth=self._max_related_depth)
 
         # Add any additional SELECTs.
         if self._select:
@@ -728,21 +734,33 @@ def get_where_clause(lookup_type, table_prefix, field_name, value):
         return backend.get_fulltext_search_sql(table_prefix + field_name)
     raise TypeError, "Got invalid lookup_type: %s" % repr(lookup_type)
 
-def get_cached_row(klass, row, index_start):
-    "Helper function that recursively returns an object with cache filled"
+def get_cached_row(klass, row, index_start, max_depth=0, cur_depth=0):
+    """Helper function that recursively returns an object with cache filled"""
+    
+    # If we've got a max_depth set and we've exceeded that depth, bail now.
+    if max_depth and cur_depth > max_depth:
+        return None
+    
     index_end = index_start + len(klass._meta.fields)
     obj = klass(*row[index_start:index_end])
     for f in klass._meta.fields:
         if f.rel and not f.null:
-            rel_obj, index_end = get_cached_row(f.rel.to, row, index_end)
+            cached_row = get_cached_row(f.rel.to, row, index_end, max_depth, cur_depth+1)
+            if cached_row:
+                rel_obj, index_end = cached_row
                 setattr(obj, f.get_cache_name(), rel_obj)
     return obj, index_end
 
-def fill_table_cache(opts, select, tables, where, old_prefix, cache_tables_seen):
+def fill_table_cache(opts, select, tables, where, old_prefix, cache_tables_seen, max_depth=0, cur_depth=0):
     """
     Helper function that recursively populates the select, tables and where (in
     place) for select_related queries.
     """
+    
+    # If we've got a max_depth set and we've exceeded that depth, bail now.
+    if max_depth and cur_depth > max_depth:
+        return None
+    
     qn = backend.quote_name
     for f in opts.fields:
         if f.rel and not f.null:
@@ -757,7 +775,7 @@ def fill_table_cache(opts, select, tables, where, old_prefix, cache_tables_seen)
             where.append('%s.%s = %s.%s' % \
                 (qn(old_prefix), qn(f.column), qn(db_table), qn(f.rel.get_related_field().column)))
             select.extend(['%s.%s' % (qn(db_table), qn(f2.column)) for f2 in f.rel.to._meta.fields])
-            fill_table_cache(f.rel.to._meta, select, tables, where, db_table, cache_tables_seen)
+            fill_table_cache(f.rel.to._meta, select, tables, where, db_table, cache_tables_seen, max_depth, cur_depth+1)
 
 def parse_lookup(kwarg_items, opts):
     # Helper function that handles converting API kwargs

docs/db-api.txt

@@ -596,6 +596,19 @@ related ``Person`` *and* the related ``City``::
 Note that ``select_related()`` does not follow foreign keys that have
 ``null=True``.
 
+Usually, using ``select_related()`` can vastly improve performance since your
+app can avoid many database calls. However, in situations with deeply nested
+sets of relationships ``select_related()`` can sometimes end up following "too
+many" relations, and can generate queries so large that they end up being slow.
+
+In these situations, you can use the ``depth`` argument to ``select_related()``
+to control how many "levels" of relations ``select_related()`` will actually
+follow::
+
+    b = Book.objects.select_related(depth=1).get(id=4)
+    p = b.author         # Doesn't hit the database.
+    c = p.hometown       # Requires a database call.
+    
 ``extra(select=None, where=None, params=None, tables=None)``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

tests/modeltests/select_related/models.py

@@ -0,0 +1,152 @@
+"""
+XXX. Tests for ``select_related()``
+
+``select_related()`` follows all relationships and pre-caches any foreign key
+values so that complex trees can be fetched in a single query. However, this
+isn't always a good idea, so the ``depth`` argument control how many "levels"
+the select-related behavior will traverse.
+"""
+
+from django.db import models
+
+# Who remembers high school biology?
+
+class Domain(models.Model):
+    name = models.CharField(maxlength=50)
+    def __str__(self):
+        return self.name
+
+class Kingdom(models.Model):
+    name = models.CharField(maxlength=50)
+    domain = models.ForeignKey(Domain)
+    def __str__(self):
+        return self.name
+
+class Phylum(models.Model):
+    name = models.CharField(maxlength=50)
+    kingdom = models.ForeignKey(Kingdom)
+    def __str__(self):
+        return self.name
+    
+class Klass(models.Model):
+    name = models.CharField(maxlength=50)
+    phylum = models.ForeignKey(Phylum)
+    def __str__(self):
+        return self.name
+    
+class Order(models.Model):
+    name = models.CharField(maxlength=50)
+    klass = models.ForeignKey(Klass)
+    def __str__(self):
+        return self.name
+
+class Family(models.Model):
+    name = models.CharField(maxlength=50)
+    order = models.ForeignKey(Order)
+    def __str__(self):
+        return self.name
+
+class Genus(models.Model):
+    name = models.CharField(maxlength=50)
+    family = models.ForeignKey(Family)
+    def __str__(self):
+        return self.name
+
+class Species(models.Model):
+    name = models.CharField(maxlength=50)
+    genus = models.ForeignKey(Genus)
+    def __str__(self):
+        return self.name
+
+def create_tree(stringtree):
+    """Helper to create a complete tree"""
+    names = stringtree.split()
+    models = [Domain, Kingdom, Phylum, Klass, Order, Family, Genus, Species]
+    assert len(names) == len(models), (names, models)
+    
+    parent = None
+    for name, model in zip(names, models):
+        try:
+            obj = model.objects.get(name=name)
+        except model.DoesNotExist:
+            obj = model(name=name)
+        if parent:
+            setattr(obj, parent.__class__.__name__.lower(), parent)
+        obj.save()
+        parent = obj
+
+__test__ = {'API_TESTS':"""
+
+# Set up.
+# The test runner sets settings.DEBUG to False, but we want to gather queries
+# so we'll set it to True here and reset it at the end of the test suite.
+>>> from django.conf import settings
+>>> settings.DEBUG = True
+
+>>> create_tree("Eukaryota Animalia Anthropoda Insecta Diptera Drosophilidae Drosophila melanogaster")
+>>> create_tree("Eukaryota Animalia Chordata Mammalia Primates Hominidae Homo sapiens")
+>>> create_tree("Eukaryota Plantae Magnoliophyta Magnoliopsida Fabales Fabaceae Pisum sativum")
+>>> create_tree("Eukaryota Fungi Basidiomycota Homobasidiomycatae Agaricales Amanitacae Amanita muscaria")
+
+>>> from django import db
+
+# Normally, accessing FKs doesn't fill in related objects:
+>>> db.reset_queries()
+>>> fly = Species.objects.get(name="melanogaster")
+>>> fly.genus.family.order.klass.phylum.kingdom.domain
+<Domain: Eukaryota>
+>>> len(db.connection.queries)
+8
+
+# However, a select_related() call will fill in those related objects without any extra queries:
+>>> db.reset_queries()
+>>> person = Species.objects.select_related().get(name="sapiens")
+>>> person.genus.family.order.klass.phylum.kingdom.domain
+<Domain: Eukaryota>
+>>> len(db.connection.queries)
+1
+
+# select_related() also of course applies to entire lists, not just items.
+# Without select_related()
+>>> db.reset_queries()
+>>> world = Species.objects.all()
+>>> [o.genus.family for o in world]
+[<Family: Drosophilidae>, <Family: Hominidae>, <Family: Fabaceae>, <Family: Amanitacae>]
+>>> len(db.connection.queries)
+9
+
+# With select_related():
+>>> db.reset_queries()
+>>> world = Species.objects.all().select_related()
+>>> [o.genus.family for o in world]
+[<Family: Drosophilidae>, <Family: Hominidae>, <Family: Fabaceae>, <Family: Amanitacae>]
+>>> len(db.connection.queries)
+1
+
+# The "depth" argument to select_related() will stop the descent at a particular level:
+>>> db.reset_queries()
+>>> pea = Species.objects.select_related(depth=1).get(name="sativum")
+>>> pea.genus.family.order.klass.phylum.kingdom.domain
+<Domain: Eukaryota>
+
+# Notice: one few query than above because of depth=1
+>>> len(db.connection.queries)
+7
+
+>>> db.reset_queries()
+>>> pea = Species.objects.select_related(depth=5).get(name="sativum")
+>>> pea.genus.family.order.klass.phylum.kingdom.domain
+<Domain: Eukaryota>
+>>> len(db.connection.queries)
+3
+
+>>> db.reset_queries()
+>>> world = Species.objects.all().select_related(depth=2)
+>>> [o.genus.family.order for o in world]
+[<Order: Diptera>, <Order: Primates>, <Order: Fabales>, <Order: Agaricales>]
+>>> len(db.connection.queries)
+5
+
+# Reset DEBUG to where we found it.
+>>> settings.DEBUG = False
+"""}