fix(303): Allow querying viewonly relationships

sqlalchemy_continuum/relationship_builder.py

@@ -259,6 +259,7 @@ def association_subquery(self, obj):
 
         :param obj: SQLAlchemy declarative object
         """
+        self.set_association_version_table(build=False)
 
         tx_column = option(obj, 'transaction_column_name')
         join_column = self.property.primaryjoin.right.name
@@ -306,12 +307,18 @@ def association_subquery(self, obj):
             ).correlate(self.local_cls, self.remote_cls)
         )
 
-    def build_association_version_tables(self):
+    def set_association_version_table(self, build=True):
         """
-        Builds many-to-many association version table for given property.
+        Set the many-to-many association version table for given property.
         Association version tables are used for tracking change history of
         many-to-many associations.
+
+        :param build:
+            Whether to build the version table if it is not found in the metadata
         """
+        if hasattr(self, 'association_version_table'):
+            return
+
         column = list(self.property.remote_side)[0]
 
         self.manager.association_tables.add(column.table)
@@ -327,7 +334,7 @@ def build_association_version_tables(self):
         else:
             table_name = builder.table_name
 
-        if table_name not in metadata.tables:
+        if table_name not in metadata.tables and build:
             self.association_version_table = table = builder()
             self.manager.association_version_tables.add(table)
         else:
@@ -351,10 +358,12 @@ def __call__(self):
             self.remote_cls = self.property.mapper.class_
 
         if (self.property.secondary is not None and
-                not self.property.viewonly and
                 not self.manager.is_excluded_property(
                     self.model, self.property.key)):
-            self.build_association_version_tables()
+            if not self.property.viewonly:
+                # viewonly properties don't need set the association_version_table at this time
+                # and are set during query time (when all tables are added to metadata)
+                self.set_association_version_table(build=True)
 
             # store remote cls to association table column pairs
             self.remote_to_association_column_pairs = []

tests/relationships/test_many_to_many_relations.py

@@ -16,6 +16,7 @@ class Article(self.Model):
 
             id = sa.Column(sa.Integer, autoincrement=True, primary_key=True)
             name = sa.Column(sa.Unicode(255))
+            is_sponsored = sa.Column(sa.Boolean(), default=False)
 
         article_tag = sa.Table(
             'article_tag',
@@ -48,6 +49,15 @@ class Tag(self.Model):
             secondary=article_tag,
             backref='tags'
         )
+        Tag.sponsored_articles = sa.orm.relationship(
+            Article,
+            secondary=article_tag,
+            secondaryjoin=sa.and_(
+                article_tag.c.article_id == Article.id,
+                Article.is_sponsored == True,
+            ),
+            viewonly=True,
+        )
 
         self.Article = Article
         self.Tag = Tag
@@ -217,6 +227,24 @@ def test_relations_with_varying_transactions(self):
         assert tag1.versions[2] in article.versions[2].tags
         assert tag2.versions[0] in article.versions[2].tags
 
+    def test_viewonly_relationships(self):
+        tag = self.Tag(name=u'some tag')
+        article1 = self.Article(
+            name=u'Sponsored Article',
+            is_sponsored=True,
+            tags=[tag],
+        )
+        article2 = self.Article(
+            name=u'Normal Article',
+            tags=[tag],
+        )
+        self.session.add(article1)
+        self.session.add(article2)
+        self.session.commit()
+
+        assert len(tag.versions[0].articles) == 2
+        assert len(tag.versions[0].sponsored_articles) == 1
+
 
 create_test_cases(ManyToManyRelationshipsTestCase)