Nested transactions with pg-promise

Question

I am using NodeJS, PostgreSQL and the amazing pg-promise library. In my case, I want to execute three main queries:

Insert one tweet in the table 'tweets'.
In case there is hashtags in the tweet, insert them into another table 'hashtags'
Them link both tweet and hashtag in a third table 'hashtagmap' (many to many relational table)

Here is a sample of the request's body (JSON):

{
"id":"12344444",
"created_at":"1999-01-08 04:05:06 -8:00",
"userid":"@postman",
"tweet":"This is the first test from postman!",
"coordinates":"",
"favorite_count":"0",
"retweet_count":"2",
"hashtags":{
    "0":{
        "name":"test",
        "relevancetraffic":"f",
        "relevancedisaster":"f"
    },
    "1":{
        "name":"postman",
        "relevancetraffic":"f",
        "relevancedisaster":"f"
    },
    "2":{
        "name":"bestApp",
        "relevancetraffic":"f",
        "relevancedisaster":"f"
    }
}

All the fields above should be included in the table "tweets" besides hashtags, that in turn should be included in the table "hashtags".

Here is the code I am using based on Nested transactions from pg-promise docs inside a NodeJS module. I guess I need nested transactions because I need to know both tweet_id and hashtag_id in order to link them in the hashtagmap table.

// Columns
var tweetCols = ['id','created_at','userid','tweet','coordinates','favorite_count','retweet_count'];

var hashtagCols = ['name','relevancetraffic','relevancedisaster'];

//pgp Column Sets
var cs_tweets = new pgp.helpers.ColumnSet(tweetCols, {table: 'tweets'});

var cs_hashtags = new pgp.helpers.ColumnSet(hashtagCols, {table:'hashtags'});
return{
// Transactions
add: body =>
    rep.tx(t => {
        return t.one(pgp.helpers.insert(body,cs_tweets)+" ON CONFLICT(id) DO UPDATE SET coordinates = "+body.coordinates+" RETURNING id")
            .then(tweet => {
                var queries = [];
                for(var i = 0; i < body.hashtags.length; i++){
                    queries.push(
                        t.tx(t1 => {
                            return t1.one(pgp.helpers.insert(body.hashtags[i],cs_hashtags) + "ON CONFLICT(name) DO UPDATE SET fool ='f' RETURNING id")
                                .then(hash =>{
                                    t1.tx(t2 =>{
                                        return t2.none("INSERT INTO hashtagmap(tweetid,hashtagid) VALUES("+tweet.id+","+hash.id+") ON CONFLICT DO NOTHING");
                                    });
                                });
                        }));
                }
                return t.batch(queries);
            });
    })
}

The problem is with this code I am being able to successfully insert the tweet but nothing happens then. I cannot insert the hashtags nor link the hashtag to the tweets.

Sorry but I am new to coding so I guess I didn't understood how to properly return from the transaction and how to perform this simple task. Hope you can help me.

Thank you in advance.

Jean

vitaly-t · Accepted Answer

Improving on Jean Phelippe's own answer:

// Columns
var tweetCols = ['id', 'created_at', 'userid', 'tweet', 'coordinates', 'favorite_count', 'retweet_count'];

var hashtagCols = ['name', 'relevancetraffic', 'relevancedisaster'];

//pgp Column Sets
var cs_tweets = new pgp.helpers.ColumnSet(tweetCols, {table: 'tweets'});

var cs_hashtags = new pgp.helpers.ColumnSet(hashtagCols, {table: 'hashtags'});

return {
    /* Tweets */
    // Add a new tweet and update the corresponding hash tags
    add: body =>
        db.tx(t => {
            return t.one(pgp.helpers.insert(body, cs_tweets) + ' ON CONFLICT(id) DO UPDATE SET coordinates = ' + body.coordinates + ' RETURNING id')
                .then(tweet => {
                    var queries = Object.keys(body.hashtags).map((_, idx) => {
                        return t.one(pgp.helpers.insert(body.hashtags[i], cs_hashtags) + 'ON CONFLICT(name) DO UPDATE SET fool = $1 RETURNING id', 'f')
                            .then(hash => {
                                return t.none('INSERT INTO hashtagmap(tweetid, hashtagid) VALUES($1, $2) ON CONFLICT DO NOTHING', [+tweet.id, +hash.id]);
                            });
                    });
                    return t.batch(queries);
                });
        })
            .then(data => {
                // transaction was committed;
                // data = [null, null,...] as per t.none('INSERT INTO hashtagmap...
            })
            .catch(error => {
                // transaction rolled back
            })
},

NOTES:

As per my notes earlier, you must chain all queries, or else you will end up with loose promises
Stay away from nested transactions, unless you understand exactly how they work in PostgreSQL (read this, and specifically the Limitations section).
Avoid manual query formatting, it is not safe, always rely on the library's query formatting.
Unless you are passing the result of transaction somewhere else, you should at least provide the .catch handler.

P.S. For the syntax like +tweet.id, it is the same as parseInt(tweet.id), just shorter, in case those are strings ;)

Nested transactions with pg-promise

Answers (2)

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