Fixed a very strange error droping field with check constraints

Related to issue 154
Tested in firebird 3.0.x

firebird/introspection.py

@@ -186,12 +186,12 @@ def get_indexes(self, cursor, table_name):
             cursor.execute("""
             SELECT
               LOWER(s.RDB$FIELD_NAME) AS field_name,
-    
+
               LOWER(case
                 when rc.RDB$CONSTRAINT_TYPE is not null then rc.RDB$CONSTRAINT_TYPE
                 else 'INDEX'
               end) AS constraint_type
-    
+
             FROM RDB$INDEX_SEGMENTS s
             LEFT JOIN RDB$INDICES i ON i.RDB$INDEX_NAME = s.RDB$INDEX_NAME
             LEFT JOIN RDB$RELATION_CONSTRAINTS rc ON rc.RDB$INDEX_NAME = s.RDB$INDEX_NAME
@@ -250,7 +250,7 @@ def get_constraints(self, cursor, table_name):
             when s.RDB$FIELD_NAME is not null then s.RDB$FIELD_NAME
             else ''
           end AS field_name,
-          
+
           i2.RDB$RELATION_NAME AS references_table,
           s2.RDB$FIELD_NAME AS references_field,
           i.RDB$UNIQUE_FLAG,
@@ -329,6 +329,21 @@ def _get_field_indexes(self, cursor, table_name, field_name):
 
         return [index_name[0].strip() for index_name in cursor.fetchall()]
 
+    def _get_check_constraints(self, cursor, table_name, field_name):
+        table = "'%s'" % table_name.upper()
+        field = "'%s'" % field_name.upper()
+        cursor.execute("""
+            select distinct rc.rdb$constraint_name as check_constraint_name
+            from rdb$relation_constraints rc
+            join rdb$check_constraints cc on rc.rdb$constraint_name = cc.rdb$constraint_name
+            join rdb$triggers c on cc.rdb$trigger_name = c.rdb$trigger_name
+            join rdb$relation_fields rf on rc.rdb$relation_name = rf.rdb$relation_name
+            where rc.rdb$relation_name = %s
+            and rf.rdb$field_name = %s
+            and rc.rdb$constraint_type = 'CHECK'
+        """ % (table, field,))
+        return [cn[0].strip() for cn in cursor.fetchall()]
+
     def _name_to_index(self, cursor, table_name):
         """Return a dictionary of {field_name: field_index} for the given table.
            Indexes are 0-based.

firebird/schema.py

@@ -47,8 +47,8 @@ def _alter_column_set_null(self, table_name, column_name, is_null):
         engine_ver = str(self.connection.connection.engine_version).split('.')
         if engine_ver and len(engine_ver) > 0 and int(engine_ver[0]) >= 3:
             sql = """
-                ALTER TABLE \"%(table_name)s\" 
-                ALTER \"%(column)s\" 
+                ALTER TABLE \"%(table_name)s\"
+                ALTER \"%(column)s\"
                 %(null_flag)s NOT NULL
             """
             null_flag = 'DROP' if is_null else 'SET'
@@ -237,6 +237,12 @@ def _get_field_indexes(self, model, field):
             indexes = self.connection.introspection._get_field_indexes(cursor, model._meta.db_table, field.column)
         return indexes
 
+    def _get_field_check_constraints(self, model, field):
+        with self.connection.cursor() as cursor:
+            db_table = model._meta.db_table
+            checks = self.connection.introspection._get_check_constraints(cursor, db_table, field.column)
+        return checks
+
     def remove_field(self, model, field):
         # If remove a AutoField, we need remove all related stuff
         # if isinstance(field, AutoField):
@@ -252,6 +258,11 @@ def remove_field(self, model, field):
             sql = self._delete_constraint_sql(self.sql_delete_index, model, index_name)
             self.execute(sql)
 
+        # If field has check constraint, then remove it first
+        for check_constraint_name in self._get_field_check_constraints(model, field):
+            sql = self._delete_constraint_sql(self.sql_delete_constraint, model, check_constraint_name)
+            self.execute(sql)
+
         super(DatabaseSchemaEditor, self).remove_field(model, field)
 
     def _alter_column_type_sql(self, table, old_field, new_field, new_type):
@@ -875,4 +886,4 @@ def execute(self, sql, params=[]):
                 except Exception as e:
                     raise e
         else:
-            super(DatabaseSchemaEditor, self).execute(sql, params)
+            super(DatabaseSchemaEditor, self).execute(sql, params)