editing a SelectMultipleField with pre-existing data in the database

Question

Am building a flask app. I have posts and tags. When making a post, one can select many tags for the post (just like here is stack overflow). The problem now comes in where the post is to be edited. Due to the pre-existing values, I get a sqlalchemy.exc.IntegrityError due to duplicate entries. I have tried removing the duplicate entries before committing to the database by

post.tags = list(set(post.tags.all()))

but it still brings the same error. Here is the view function for editing the post:

@home.route('/edit/<int:id>', methods=['GET', 'POST'])
@login_required
def edit_post(id):
    post = Post.query.get_or_404(id)
    if current_user != post.author and not current_user.can(Permission.ADMINISTER):
        abort(403)
    form = PostForm()
    form.tag.choices = [(tag.id, tag.name) for tag in Tag.query.order_by('name')]
    if form.validate_on_submit():
        post.body = form.body.data
        for id in form.tag.data:
            post.tags.append(Tag.query.get(id))
        db.session.commit()
        return redirect(url_for('.view_post', id=post.id))
    form.body.data = post.body
    form.tag.choices = [(tag.id, tag.name) for tag in Tag.query.order_by('name')]
    form.tag.data = post.tags.all()
    return render_template('home/edit_post.html', form=form, title='Edit Post')

Kindly help me solve this or advise me on a better logic of doing this. Consider me a beginner.

Answer 1

After numerous trials, I was able to implement the editing successfully. So first I queried for all categories in this table filtering with the post id (returns a list of tuples) eg [(62, 1), (62, 4)]. The tuple is (post_id, tag_id).

categories = db.Table('categories', db.Column('post_id', db.Integer, db.ForeignKey('posts.id'), primary_key=True), db.Column('tag_id', db.Integer, db.ForeignKey('tags.id'), primary_key=True))

Then I custom made a tuple which I used to implement an if condition where if that tuple(s) exists in the list from categories, it is ignored and only the unique one is committed to the database.

Here is the new view function:

@home.route('/edit/<int:id>', methods=['GET', 'POST'])
@login_required
def edit_post(id):
    post = Post.query.get_or_404(id)
    if current_user != post.author and not current_user.can(Permission.ADMINISTER):
        abort(403)

    form = PostForm()
    form.tag.choices = [(tag.id, tag.name) for tag in Tag.query.order_by('name')]

    if form.validate_on_submit():
        post.body = form.body.data
        # query the post-tag relationship for this post id
        posttags = db.session.query(categories).filter_by(post_id=id).all()
        for id in form.tag.data:
            # custom tuple
            if (post.id, id) not in posttags:
                post.tags.append(Tag.query.get(id))
        db.session.commit()
        return redirect(url_for('.view_post', id=post.id))

    form.body.data = post.body
    form.tag.choices = [(tag.id, tag.name) for tag in Tag.query.order_by('name')]
    form.tag.data = post.tags.all()
    return render_template('home/edit_post.html', form=form, title='Edit Post')