Fixed #12990, Refs #27694 -- Added JSONField model field.

Thanks to Adam Johnson, Carlton Gibson, Mariusz Felisiak, and Raphael
Michel for mentoring this Google Summer of Code 2019 project and
everyone else who helped with the patch.

Special thanks to Mads Jensen, Nick Pope, and Simon Charette for
extensive reviews.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

django/db/backends/sqlite3/introspection.py

@@ -9,7 +9,7 @@ from django.db.backends.base.introspection import (
 from django.db.models import Index
 from django.utils.regex_helper import _lazy_re_compile
 
-FieldInfo = namedtuple('FieldInfo', BaseFieldInfo._fields + ('pk',))
+FieldInfo = namedtuple('FieldInfo', BaseFieldInfo._fields + ('pk', 'has_json_constraint'))
 
 field_size_re = _lazy_re_compile(r'^\s*(?:var)?char\s*\(\s*(\d+)\s*\)\s*$')
 
@@ -63,6 +63,8 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             # No support for BigAutoField or SmallAutoField as SQLite treats
             # all integer primary keys as signed 64-bit integers.
             return 'AutoField'
+        if description.has_json_constraint:
+            return 'JSONField'
         return field_type
 
     def get_table_list(self, cursor):
@@ -81,12 +83,28 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         interface.
         """
         cursor.execute('PRAGMA table_info(%s)' % self.connection.ops.quote_name(table_name))
+        table_info = cursor.fetchall()
+        json_columns = set()
+        if self.connection.features.can_introspect_json_field:
+            for line in table_info:
+                column = line[1]
+                json_constraint_sql = '%%json_valid("%s")%%' % column
+                has_json_constraint = cursor.execute("""
+                    SELECT sql
+                    FROM sqlite_master
+                    WHERE
+                        type = 'table' AND
+                        name = %s AND
+                        sql LIKE %s
+                """, [table_name, json_constraint_sql]).fetchone()
+                if has_json_constraint:
+                    json_columns.add(column)
         return [
             FieldInfo(
                 name, data_type, None, get_field_size(data_type), None, None,
-                not notnull, default, pk == 1,
+                not notnull, default, pk == 1, name in json_columns
             )
-            for cid, name, data_type, notnull, default, pk in cursor.fetchall()
+            for cid, name, data_type, notnull, default, pk in table_info
         ]
 
     def get_sequences(self, cursor, table_name, table_fields=()):