Support Django serialization framework, and dumpdata/loaddata

This required adding BaseField.value_to_string() and
BaseField.to_python() methods.

Author: Tim Heap

postgres_composite_types/__init__.py

@@ -35,6 +35,7 @@
 """
 
 import inspect
+import json
 import logging
 import sys
 from collections import OrderedDict
@@ -132,6 +133,36 @@ def formfield(self, **kwargs):  # pylint:disable=arguments-differ
 
         return super().formfield(**defaults)
 
+    def to_python(self, value):
+        """
+        Convert a value to the correct type for this field. Values from the
+        database will already be of the correct type, due to the the caster
+        registered with psycopg2. The field can also be serialized as a string
+        via value_to_string, where it is encoded as a JSON object.
+        """
+        # Composite types are serialized as JSON blobs. If BaseField.to_python
+        # is called with a string, assume it was produced by value_to_string
+        # and decode it
+        if isinstance(value, str):
+            value = json.loads(value)
+            return self.Meta.model(**{
+                name: field.to_python(value.get(name))
+                for name, field in self.Meta.fields
+            })
+
+        return super().to_python(value)
+
+    def value_to_string(self, obj):
+        """
+        Serialize this as a JSON object {name: field.value_to_string(...)} for
+        each child field.
+        """
+        value = self.value_from_object(obj)
+        return json.dumps({
+            name: field.value_to_string(value)
+            for name, field in self.Meta.fields
+        })
+
 
 class BaseOperation(migrations.operations.base.Operation):
     """Base class for the DB operation that relates to this type."""

tests/test_field.py

@@ -3,6 +3,7 @@
 import datetime
 from unittest import mock
 
+from django.core import serializers
 from django.db import connection
 from django.db.migrations.executor import MigrationExecutor
 from django.test import TestCase, TransactionTestCase
@@ -148,6 +149,21 @@ def test_adapted_sql(self):
             b"(1, 'b', '1985-10-26T09:00:00'::timestamp)::test_type",
             adapted.getquoted())
 
+    def test_serialize(self):
+        """
+        Check that composite values are correctly handled through Django's
+        serialize/deserialize helpers, used for dumpdata/loaddata.
+        """
+        old = Item(
+            name="table",
+            bounding_box=Box(top_left=Point(x=1, y=1),
+                             bottom_right=Point(x=4, y=2)))
+        out = serializers.serialize("json", [old])
+        new = next(serializers.deserialize("json", out)).object
+
+        self.assertEqual(old.bounding_box,
+                         new.bounding_box)
+
 
 class TestOptionalFields(TestCase):
     """