Added introspection of table comments.

django/db/backends/base/introspection.py

@@ -77,6 +77,13 @@ def get_table_description(self, cursor, table_name):
             "get_table_description() method."
         )
 
+    def get_table_comment(self, cursor, table_name):
+        """Return a comment on the table."""
+        raise NotImplementedError(
+            "subclasses of BaseDatabaseIntrospection may require a get_table_comment() "
+            "method."
+        )
+
     def get_migratable_models(self):
         from django.apps import apps
         from django.db import router

django/db/backends/mysql/introspection.py

@@ -166,6 +166,19 @@ def to_int(i):
             )
         return fields
 
+    def get_table_comment(self, cursor, table_name):
+        cursor.execute(
+            """
+            SELECT table_comment
+            FROM information_schema.tables
+            WHERE table_schema = DATABASE()
+                AND table_name = %s
+            """,
+            [table_name],
+        )
+        row = cursor.fetchone()
+        return row[0] if row else None
+
     def get_sequences(self, cursor, table_name, table_fields=()):
         for field_info in self.get_table_description(cursor, table_name):
             if "auto_increment" in field_info.extra:

django/db/backends/oracle/introspection.py

@@ -201,6 +201,18 @@ def get_table_description(self, cursor, table_name):
             )
         return description
 
+    def get_table_comment(self, cursor, table_name):
+        cursor.execute(
+            """
+            SELECT comments
+            FROM user_tab_comments
+            WHERE table_name = UPPER(%s)
+            """,
+            [table_name],
+        )
+        row = cursor.fetchone()
+        return row[0] if row else None
+
     def identifier_converter(self, name):
         """Identifier comparison is case insensitive under Oracle."""
         return name.lower()

django/db/backends/postgresql/introspection.py

@@ -124,6 +124,22 @@ def get_table_description(self, cursor, table_name):
             for line in cursor.description
         ]
 
+    def get_table_comment(self, cursor, table_name):
+        cursor.execute(
+            """
+            SELECT obj_description(pg_class.oid)
+            FROM pg_class
+            JOIN pg_namespace ON pg_class.relnamespace = pg_namespace.oid
+            WHERE pg_class.relkind IN ('f', 'm', 'p', 'r', 'v')
+                AND pg_class.relname = %s
+                AND pg_namespace.nspname NOT IN ('pg_catalog', 'pg_toast')
+                AND pg_catalog.pg_table_is_visible(pg_class.oid)
+            """,
+            [table_name],
+        )
+        row = cursor.fetchone()
+        return row[0] if row else None
+
     def get_sequences(self, cursor, table_name, table_fields=()):
         cursor.execute(
             """

tests/backends/base/test_introspection.py

@@ -21,6 +21,11 @@ def test_get_table_description(self):
         with self.assertRaisesMessage(NotImplementedError, msg):
             self.introspection.get_table_description(None, None)
 
+    def test_get_table_comment(self):
+        msg = self.may_require_msg % "get_table_comment"
+        with self.assertRaisesMessage(NotImplementedError, msg):
+            self.introspection.get_table_comment(None, None)
+
     def test_get_sequences(self):
         msg = self.may_require_msg % "get_sequences"
         with self.assertRaisesMessage(NotImplementedError, msg):

tests/introspection/models.py

@@ -55,16 +55,6 @@ class Meta:
         managed = False
 
 
-class ArticleReporterWithDBComment(models.Model):
-    article = models.ForeignKey(Article, models.CASCADE, db_comment="i am comment")
-    reporter = models.ForeignKey(Reporter, models.CASCADE)
-
-    class Meta:
-        managed = False
-        db_table_comment = "I am table comment"
-        required_db_features = {"supports_comments"}
-
-
 class Comment(models.Model):
     ref = models.UUIDField(unique=True)
     article = models.ForeignKey(Article, models.CASCADE, db_index=True)
@@ -112,3 +102,11 @@ class Meta:
                 condition=models.Q(color__isnull=True),
             ),
         ]
+
+
+class DbCommentModel(models.Model):
+    name = models.CharField(max_length=15, db_comment="'Name' column comment")
+
+    class Meta:
+        db_table_comment = "Custom table comment"
+        required_db_features = {"supports_comments"}

tests/introspection/tests.py

@@ -5,11 +5,11 @@
 from .models import (
     Article,
     ArticleReporter,
-    ArticleReporterWithDBComment,
     CheckConstraintModel,
     City,
     Comment,
     Country,
+    DbCommentModel,
     District,
     Reporter,
     UniqueConstraintConditionModel,
@@ -80,11 +80,6 @@ def test_unmanaged_through_model(self):
         tables = connection.introspection.django_table_names()
         self.assertNotIn(ArticleReporter._meta.db_table, tables)
 
-    @skipUnlessDBFeature("supports_comments", "supports_foreign_keys")
-    def test_unmanaged_db_comment_model(self):
-        tables = connection.introspection.django_table_names()
-        self.assertNotIn(ArticleReporterWithDBComment._meta.db_table, tables)
-
     def test_installed_models(self):
         tables = [Article._meta.db_table, Reporter._meta.db_table]
         models = connection.introspection.installed_models(tables)
@@ -185,6 +180,21 @@ def test_smallautofield(self):
             [connection.introspection.get_field_type(r[1], r) for r in desc],
         )
 
+    @skipUnlessDBFeature("supports_comments")
+    def test_db_comments(self):
+        with connection.cursor() as cursor:
+            desc = connection.introspection.get_table_description(
+                cursor, DbCommentModel._meta.db_table
+            )
+            table_comment = connection.introspection.get_table_comment(
+                cursor, DbCommentModel._meta.db_table
+            )
+        self.assertEqual(
+            ["'Name' column comment"],
+            [field.comment for field in desc if field.name == "name"],
+        )
+        self.assertEqual(table_comment, "Custom table comment")
+
     # Regression test for #9991 - 'real' types in postgres
     @skipUnlessDBFeature("has_real_datatype")
     def test_postgresql_real_type(self):