Fixed #26441 -- Added model Field.db_check() method

Thanks Common Code for financing the work on this commit.

django/db/models/fields/__init__.py

@@ -594,9 +594,21 @@ class Field(RegisterLookupMixin):
         self.run_validators(value)
         return value
 
+    def db_check(self, connection):
+        """
+        Return the database column check constraint for this field, for the
+        provided connection. Works the same way as db_type() for the case that
+        get_internal_type() does not map to a preexisting model field.
+        """
+        data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
+        try:
+            return connection.data_type_check_constraints[self.get_internal_type()] % data
+        except KeyError:
+            return None
+
     def db_type(self, connection):
         """
-        Returns the database column data type for this field, for the provided
+        Return the database column data type for this field, for the provided
         connection.
         """
         # The default implementation of this method looks at the
@@ -634,12 +646,8 @@ class Field(RegisterLookupMixin):
         values (type, checks).
         This will look at db_type(), allowing custom model fields to override it.
         """
-        data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
         type_string = self.db_type(connection)
-        try:
-            check_string = connection.data_type_check_constraints[self.get_internal_type()] % data
-        except KeyError:
-            check_string = None
+        check_string = self.db_check(connection)
         return {
             "type": type_string,
             "check": check_string,

django/db/models/fields/related.py

@@ -949,11 +949,14 @@ class ForeignKey(ForeignObject):
         defaults.update(kwargs)
         return super(ForeignKey, self).formfield(**defaults)
 
+    def db_check(self, connection):
+        return []
+
     def db_type(self, connection):
         return self.target_field.rel_db_type(connection=connection)
 
     def db_parameters(self, connection):
-        return {"type": self.db_type(connection), "check": []}
+        return {"type": self.db_type(connection), "check": self.db_check(connection)}
 
     def convert_empty_strings(self, value, expression, connection, context):
         if (not value) and isinstance(value, six.string_types):
@@ -1614,6 +1617,9 @@ class ManyToManyField(RelatedField):
             defaults['initial'] = [i._get_pk_val() for i in initial]
         return super(ManyToManyField, self).formfield(**defaults)
 
+    def db_check(self, connection):
+        return None
+
     def db_type(self, connection):
         # A ManyToManyField is not represented by a single column,
         # so return None.