How to enter value for foriegn key field via sqlalchemy

Question

I have models:

class Post(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    body = db.Column(db.String(2000))
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    comments = db.relationship('Comment', backref='parent_post', lazy='dynamic')


class Comment(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    body = db.Column(db.String(140))
    post_id = db.Column(db.Integer, db.ForeignKey('post.id'))

When entering a post to database I do this:

if form.validate_on_submit():
      post = Post(body=form.post.data, author=g.user)
      db.session.add(post)
      db.session.commit()

This is working right.

But how can I enter a comment to database if I want to pass the 'post.id' value directly instead of object 'post'. (Not able to 'pass' object via form in html)

if form.validate_on_submit():
      comment = Comment(body=form.post.data, parent_post=form.p_id.data)
      db.session.add(comment)
      db.session.commit()

currently p_id holds value post.id and it gives me error:

AttributeError: 'int' object has no attribute '_sa_instance_state'

Answer 1

Comment.parent_post is a relationship, backed by the integer column Comment.post_id. Currently, you are trying to assign an int (from form.p_id) too the relationship. Assign an int to the column or a Post instance to the relationship.

comment = Comment(post_id=form.p_id.data, body=form.post.data)
# or
post = Post.query.get_or_404(form.p_id.data)
comment = Comment(parent_post=post, body=form.post.data)

The second way is preferable, because you validate that a post with the id exists before trying to use the id.

Answer 2

if form.validate_on_submit():
      comment = Comment(body=form.post.data, parent_post=form.p_id.data)
      ### you need to add `comment` instead `post` in the line below
      db.session.add(comment)
      db.session.commit()

I strongly think that an error was because of the db.session.add(post). Please the line db.session.add(post) with db.session.add(comment) while adding the comments.

Please make sure that POST-ID that being passed through the form is existing in the post table.