add user_corpus m2m relationship

app/models.py

@@ -257,7 +257,7 @@ class Avatar(HashidMixin, FileMixin, db.Model):
     # Primary key
     id = db.Column(db.Integer, primary_key=True)
     # Foreign keys
-    user_id = db.Column(db.Integer, db.ForeignKey('users.id'), unique=True)
+    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))
     
     @property
     def path(self):
@@ -276,7 +276,16 @@ class Avatar(HashidMixin, FileMixin, db.Model):
             **self.file_mixin_to_json_serializeable()
         }
         return json_serializeable
-    
+
+
+corpus_followers = db.Table(
+    'corpus_followers',
+    db.Model.metadata,
+    db.Column('user_id', db.ForeignKey('users.id'), primary_key=True),
+    db.Column('corpus_id', db.ForeignKey('corpora.id'), primary_key=True)
+)
+
+
 class User(HashidMixin, UserMixin, db.Model):
     __tablename__ = 'users'
     # Primary key
@@ -327,6 +336,13 @@ class User(HashidMixin, UserMixin, db.Model):
         cascade='all, delete-orphan',
         lazy='dynamic'
     )
+    followed_corpora = db.relationship(
+        'Corpus',
+        secondary=corpus_followers,
+        primaryjoin=(corpus_followers.c.user_id == id),
+        backref=db.backref('followers', lazy='dynamic'),
+        lazy='dynamic'
+    )
     jobs = db.relationship(
         'Job',
         backref='user',