Fix issues with delete-cascade

The important tables `xml` and `registration` were not properly set for their `CASCADE` behavior, in addiiton `XML` needed to have the `single_parent` option enabled to allow for cascading-deletes (since otherwise a single entry could be referenced by an entry and a error.

model/cce.py

@@ -39,7 +39,11 @@ class CCE(Core, Base):
 
     volume_id = Column(Integer, ForeignKey('volume.id'))
 
-    registrations = relationship('Registration', backref='cce')
+    registrations = relationship(
+        'Registration',
+        backref='cce',
+        cascade='all, delete-orphan'
+    )
     lccns = relationship('LCCN', backref='cce', cascade='all, delete-orphan')
     authors = relationship('Author',
                            backref='cce', cascade='all, delete-orphan')

model/xml.py

@@ -14,9 +14,11 @@ from sqlalchemy import (
 
 from model.core import Base, Core
 
+
 @compiles(String, 'postgresql')
 def compile_xml(type_, compiler, **kw):
-    return "XML"
+    return 'XML'
+
 
 ENTRY_XML = Table(
     'entry_xml',
@@ -32,19 +34,24 @@ ERROR_XML = Table(
     Column('xml_id', Integer, ForeignKey('xml.id'), index=True)
 )
 
+
 class XML(Core, Base):
     __tablename__ = 'xml'
     id = Column(Integer, primary_key=True)
     xml_source = Column(String)
-    
+
     entry = relationship(
         'CCE',
         secondary=ENTRY_XML,
-        backref='xml_sources'
+        backref='xml_sources',
+        single_parent=True,
+        cascade='all, delete-orphan'
     )
 
     error_entry = relationship(
         'ErrorCCE',
         secondary=ERROR_XML,
-        backref='xml_sources'
+        backref='xml_sources',
+        single_parent=True,
+        cascade='all, delete-orphan'
     )