Add optimized ToOne sqlalchemy aware field.

This version of fields.ToOne() is able to read the local foreign key value
to serialize the reference of the remote object.
It can be useful under some circumstances to avoid performing additional db lookups.

flask_potion/contrib/alchemy/fields.py

@@ -1,4 +1,7 @@
-from flask_potion.fields import Object
+from werkzeug.utils import cached_property
+
+from flask_potion.fields import Object, ToOne as GenericToOne
+from flask_potion.utils import get_value, route_from
 
 
 class InlineModel(Object):
@@ -15,3 +18,27 @@ def converter(self, instance):
         if instance is not None:
             instance = self.model(**instance)
         return instance
+
+
+class ToOne(GenericToOne):
+    """
+    Same as flask_potion.fields.ToOne
+    except it will use the local id stored on the ForeignKey field to serialize the field.
+    This is an optimisation to avoid additional lookups to the database,
+    in order to prevent fetching the remote object, just to obtain its id,
+    that we already have.
+    Limitations:
+    - It works only if the foreign key is made of a single field.
+    - It works only if the serialization is using the ForeignKey as source of information to Identify the remote resource.
+    - `attribute` parameter is ignored.
+    """
+    def output(self, key, obj):
+        column = getattr(obj.__class__, key)
+        local_columns = column.property.local_columns
+        assert len(local_columns) == 1
+        local_column = list(local_columns)[0]
+        key = local_column.key
+        return self.format(get_value(key, obj, self.default))
+
+    def formatter(self, item):
+        return self.formatter_key.format(item, is_local=True)

flask_potion/natural_keys.py

@@ -11,6 +11,7 @@
 
 
 class Key(Schema, ResourceBound):
+    is_local = False
 
     def matcher_type(self):
         type_ = self.response['type']
@@ -43,11 +44,16 @@ def schema(self):
             "additionalProperties": False
         }
 
+    def _id_uri(self, resource, id_):
+        return '{}/{}'.format(resource.route_prefix, id_)
+
     def _item_uri(self, resource, item):
         # return url_for('{}.instance'.format(self.resource.meta.id_attribute, item, None), get_value(self.resource.meta.id_attribute, item, None))
         return '{}/{}'.format(resource.route_prefix, get_value(resource.manager.id_attribute, item, None))
 
-    def format(self, item):
+    def format(self, item, is_local=False):
+        if is_local:
+            return {'$ref': self._id_uri(self.resource, item)}
         return {"$ref": self._item_uri(self.resource, item)}
 
     def convert(self, value):
@@ -71,7 +77,7 @@ def rebind(self, resource):
     def schema(self):
         return self.resource.schema.fields[self.property].request
 
-    def format(self, item):
+    def format(self, item, is_local=False):
         return self.resource.schema.fields[self.property].output(self.property, item)
 
     @cached_property
@@ -101,7 +107,7 @@ def schema(self):
             "additionalItems": False
         }
 
-    def format(self, item):
+    def format(self, item, is_local=False):
         return [self.resource.schema.fields[p].output(p, item) for p in self.properties]
 
     @cached_property
@@ -123,7 +129,7 @@ def _on_bind(self, resource):
     def schema(self):
         return self.id_field.request
 
-    def format(self, item):
+    def format(self, item, is_local=False):
         return self.id_field.output(self.resource.manager.id_attribute, item)
 
     def convert(self, value):

tests/__init__.py

@@ -1,6 +1,9 @@
+from pprint import pformat
+
 from flask import json, Flask
 from flask.testing import FlaskClient
 from flask_testing import TestCase
+import sqlalchemy
 
 
 class ApiClient(FlaskClient):
@@ -49,4 +52,52 @@ def create_app(self):
         return app
 
     def pp(self, obj):
-        print(json.dumps(obj, sort_keys=True, indent=4, separators=(',', ': ')))
+        print(json.dumps(obj, sort_keys=True, indent=4, separators=(',', ': ')))
+
+
+class DBQueryCounter:
+    """
+    Use as a context manager to count the number of execute()'s performed
+    against the given sqlalchemy connection.
+
+    Usage:
+        with DBQueryCounter(db.session) as ctr:
+            db.session.execute("SELECT 1")
+            db.session.execute("SELECT 1")
+        ctr.assert_count(2)
+    """
+
+    def __init__(self, session, reset=True):
+        self.session = session
+        self.reset = reset
+        self.statements = []
+
+    def __enter__(self):
+        if self.reset:
+            self.session.expire_all()
+        sqlalchemy.event.listen(
+            self.session.get_bind(), 'after_execute', self._callback
+        )
+        return self
+
+    def __exit__(self, *_):
+        sqlalchemy.event.remove(
+            self.session.get_bind(), 'after_execute', self._callback
+        )
+
+    def get_count(self):
+        return len(self.statements)
+
+    def _callback(self, conn, clause_element, multiparams, params, result):
+        self.statements.append((clause_element, multiparams, params))
+
+    def display_all(self):
+        for clause, multiparams, params in self.statements:
+            print(pformat(str(clause)), multiparams, params)
+            print('\n')
+        count = self.get_count()
+        return f'Counted: {count}'
+
+    def assert_count(self, expected):
+        count = self.get_count()
+        assert count == expected, self.display_all()

tests/contrib/alchemy/test_fields.py

@@ -0,0 +1,62 @@
+from flask_sqlalchemy import SQLAlchemy
+
+from flask_potion import Api, fields
+from flask_potion.resource import ModelResource
+from flask_potion.contrib.alchemy.fields import ToOne as SAToOne
+from tests import BaseTestCase, DBQueryCounter
+
+
+class SQLAlchemyToOneRemainNoPrefetchTestCase(BaseTestCase):
+    """
+    """
+
+    def setUp(self):
+        super(SQLAlchemyToOneRemainNoPrefetchTestCase, self).setUp()
+        self.app.config['SQLALCHEMY_ENGINE'] = 'sqlite://'
+        self.api = Api(self.app)
+        self.sa = sa = SQLAlchemy(
+            self.app, session_options={"autoflush": False})
+
+        class Type(sa.Model):
+            id = sa.Column(sa.Integer, primary_key=True)
+            name = sa.Column(sa.String(60), nullable=False)
+
+        class Machine(sa.Model):
+            id = sa.Column(sa.Integer, primary_key=True)
+            name = sa.Column(sa.String(60), nullable=False)
+
+            type_id = sa.Column(sa.Integer, sa.ForeignKey(Type.id))
+            type = sa.relationship(Type, foreign_keys=[type_id])
+
+        sa.create_all()
+
+        class MachineResource(ModelResource):
+            class Meta:
+                model = Machine
+
+            class Schema:
+                type = SAToOne('type')
+
+        class TypeResource(ModelResource):
+            class Meta:
+                model = Type
+
+        self.MachineResource = MachineResource
+        self.TypeResource = TypeResource
+
+        self.api.add_resource(MachineResource)
+        self.api.add_resource(TypeResource)
+
+    def test_relation_serialization_does_not_load_remote_object(self):
+        response = self.client.post('/type', data={"name": "aaa"})
+        aaa_uri = response.json["$uri"]
+        self.client.post(
+            '/machine', data={"name": "foo", "type": {"$ref": aaa_uri}})
+        with DBQueryCounter(self.sa.session) as counter:
+            response = self.client.get('/machine')
+            self.assert200(response)
+            self.assertJSONEqual(
+                [{'$uri': '/machine/1', 'type': {'$ref': aaa_uri}, 'name': 'foo'}],
+                response.json)
+        counter.assert_count(1)
+