SQLAlchemy Many-To-Many join
First, in your products table, your second foreign key needs to property reference the table name "tags", so change 'tag.id' to 'tags.id'
products_tags_table=db.Table('products_tags', db.Column('product_id', db.Integer,db.ForeignKey('products.id'), nullable=False), db.Column('tag_id',db.Integer,db.ForeignKey('tags.id'),nullable=False), #^Missing 's' db.PrimaryKeyConstraint('product_id', 'tag_id') )And then in your products model, you want to accurately reference the name of the appropriate model. Change 'ProductTag' to 'Tag'
tags=db.relationship('Tag', # 'ProductTag' <- This is a relationship to the Tag table. secondary=products_tags_table, backref='tag_products' ) Then try your join like this
query = Tag.query.join(Product, Tag.products)avail_tags = query.order_by(Tag.name).all()