Interdependent Transactions with pg-promise

I am trying to build an app involves posts and tags for posts. For these I have a post, tags and post_tag table. tags has the tags I have defined before hand and in somewhere in the app is suggested to the user on the front-end. post_tag table holds the post and tag ids as pairs on each row.

I use express.js and postgreql and pg-promise.

As far as I know I need a transactional query(ies) for a create post operation.

Also I need a mechanism to detect if a tag was not in tags table when the user created the post, so that I can insert it on the fly, and I have a tag_id for each tag that is neccessary to use in insertion of the post_id and tag_id into post_tag table. Otherwise, I will have a foreign key error since I need to post_tag table's columns post_id and tag_id to reference posts and tags table id columns, respectively.

Here is the url function I use for this I have used so far unsuccessful:

privateAPIRoutes.post('/ask', function (req, res) {
    console.log('/ask req.body: ', req.body);
    // write to posts
    var post_id = ''
    var post_url = ''
    db.query(
        `
            INSERT INTO
                posts (title, text, post_url, author_id, post_type)
            VALUES
                ($(title), $(text), $(post_url), $(author_id), $(post_type))
            RETURNING id
        `,
        {
            title: req.body.title,
            text: req.body.text,
            post_url: slug(req.body.title),
            author_id: req.user.id,
            post_type: 'question'
        } // remember req.user contains decoded jwt saved by mw above.
    )
        .then(post => {
            console.log('/ask post: ', post);
            post_id = post.id
            post_url = post.post_url


            // if tag deos not exist create it here
            var tags = req.body.tags;
            console.log('2nd block tags1', tags);
            for (var i = 0; i < tags.length; i++) {
                if (tags[i].id == undefined) {
                    console.log('req.body.tags[i].id == undefined', tags[i].id);                        
                    var q1 = db.query("insert into tags (tag) values ($(tag)) returning id", {tag: tags[i].label})
                                .then(data => {
                                    console.log('2nd block tags2', tags);
                                    tags[i].id = data[0].id 


                                    // write to the post_tag
                                    db.tx(t => {
                                        var queries = [];
                                        for (var j = 0; j < tags.length; j++) {

                                            var query = t.query(
                                                `
                                                    INSERT INTO
                                                        post_tag (post_id, tag_id)
                                                    VALUES
                                                        ($(post_id), $(tag_id))
                                                `,
                                                {
                                                    post_id: post_id,
                                                    tag_id: tags[j].id
                                                }
                                            )
                                            queries.push(query);
                                        }   
                                        return t.batch(queries)
                                    })
                                        .then(data => {
                                            res.json({post_id: post_id, post_url: post_url})
                                        })
                                        .catch(error => {
                                            console.error(error);
                                        })
                                })
                                .catch(error => {
                                    console.error(error);
                                });
                }
            }
        })
        .catch(error => {
            console.error(error);
        })
});

The main problem you have - you can't use the root-level db object inside a task or transaction. Trying to create a new connection while inside a transaction breaks the transaction logic. You would need to use t.tx in such cases. However, in your case I don't see that you need it at all.

corrected code:

privateAPIRoutes.post('/ask', (req, res) => {
    console.log('/ask req.body: ', req.body);
    db.tx(t => {
        return t.one(
            `
        INSERT INTO
        posts (title, text, post_url, author_id, post_type)
        VALUES
        ($(title), $(text), $(post_url), $(author_id), $(post_type))
        RETURNING *
        `,
            {
                title: req.body.title,
                text: req.body.text,
                post_url: slug(req.body.title),
                author_id: req.user.id,
                post_type: 'question'
            } // remember req.user contains decoded jwt saved by mw above.
        )
            .then(post => {
                console.log('/ask second query: post[0]: ', post);
                console.log('/ask second query: tags: ', req.body.tags);
                console.log('/ask second query: tags[0]: ', req.body.tags[0]);

                // the key piece to the answer:
                var tagIds = req.body.tags.map(tag => {
                    return tag.id || t.one("insert into tags(tag) values($1) returning id", tag.label, a=>a.id);
                });

                return t.batch(tagIds)
                    .then(ids => {
                        var queries = ids.map(id => {
                            return t.one(
                                `
                                INSERT INTO post_tag (post_id, tag_id)
                                VALUES ($(post_id), $(tag_id))
                                RETURNING post_id, tag_id
                                `,
                                {
                                    post_id: post.id,
                                    tag_id: id
                                }
                            )
                        });
                        return t.batch(queries);
                    });
            });
    })
        .then(data => {
            // data = result from the last query;
            console.log('/api/ask', data);
            res.json(data);

        })
        .catch(error => {
            // error
        });
});

The key here is simply to iterate through the tag id-s, and for the ones that are not set - use an insert. Then you settle them all by passing the array into t.batch.

Other recommendations:

You should use method one when executing an insert that returns the new record columns.
You should use try/catch only once there, on the transaction. This is relevant to how to use promises, and not just for this library
You can place your queries into external SQL files, see Query Files

To understand conditional inserts better, see SELECT->INSERT