Add simple JSONTextDict data type for SQLAlchemy columns

for basic JSON awareness/conversion

rattail/db/types.py

@@ -1,8 +1,8 @@
-# -*- coding: utf-8 -*-
+# -*- coding: utf-8; -*-
 ################################################################################
 #
 #  Rattail -- Retail Software Framework
-#  Copyright © 2010-2017 Lance Edgar
+#  Copyright © 2010-2019 Lance Edgar
 #
 #  This file is part of Rattail.
 #
@@ -24,7 +24,9 @@
 Data Types
 """
 
-from __future__ import unicode_literals
+from __future__ import unicode_literals, absolute_import
+
+import json
 
 from sqlalchemy import types
 
@@ -47,3 +49,23 @@ class GPCType(types.TypeDecorator):
         if value is None:
             return None
         return GPC(value)
+
+
+class JSONTextDict(types.TypeDecorator):
+    """
+    SQLAlchemy type engine for JSON data.  Interprets "raw" text as JSON when
+    reading from DB, and writes JSON text back to DB.
+
+    Note that the Python value for a field with this type will appear as a dict.
+    """
+    impl = types.Text
+
+    def process_bind_param(self, value, dialect):
+        if value is not None:
+            value = json.dumps(value)
+        return value
+
+    def process_result_value(self, value, dialect):
+        if value is not None:
+            value = json.loads(value)
+        return value