Update get_constraints with better comments

django/db/backends/__init__.py

@@ -1328,9 +1328,20 @@ class BaseDatabaseIntrospection(object):
 
     def get_constraints(self, cursor, table_name):
         """
-        Returns {'cnname': {'columns': set(columns), 'primary_key': bool, 'unique': bool}}
+        Retrieves any constraints or keys (unique, pk, fk, check, index)
+        across one or more columns.
 
-        Both single- and multi-column constraints are introspected.
+        Returns a dict mapping constraint names to their attributes,
+        where attributes is a dict with keys:
+         * columns: List of columns this covers
+         * primary_key: True if primary key, False otherwise
+         * unique: True if this is a unique constraint, False otherwise
+         * foreign_key: (table, column) of target, or None
+         * check: True if check constraint, False otherwise
+         * index: True if index, False otherwise.
+
+        Some backends may return special constraint names that don't exist
+        if they don't name constraints of a certain type (e.g. SQLite)
         """
         raise NotImplementedError
 

django/db/backends/postgresql_psycopg2/introspection.py

@@ -169,7 +169,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                     "columns": [],
                     "primary_key": False,
                     "unique": False,
-                    "foreign_key": False,
+                    "foreign_key": None,
                     "check": True,
                     "index": False,
                 }
@@ -197,7 +197,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
                     "columns": list(columns),
                     "primary_key": primary,
                     "unique": unique,
-                    "foreign_key": False,
+                    "foreign_key": None,
                     "check": False,
                     "index": True,
                 }