Add explicit DocumentNode definition

gargantext/models/nodes.py

@@ -6,7 +6,9 @@ from .base import Base, Column, ForeignKey, relationship, TypeDecorator, Index,
                   flag_modified
 from .users import User
 
-__all__ = ['Node', 'NodeNode', 'CorpusNode']
+
+__all__ = ['Node', 'NodeNode', 'CorpusNode', 'DocumentNode']
+
 
 class NodeType(TypeDecorator):
     """Define a new type of column to describe a Node's type.
@@ -124,6 +126,10 @@ class Node(ValidatorMixin, Base):
         flag_modified(self, 'hyperdata')
 
 
+class DocumentNode(Node):
+    __mapper_args__ = {
+        'polymorphic_identity': 'DOCUMENT'
+    }
 
 
 class CorpusNode(Node):