Array of queries for `for await` loop for postgresql transaction helper

Question

I made a transaction function that simplifies this action for me like this (it working):

export async function transaction(queriesRaw) {
  let allResults = []

  const client = await pool.connect()

  try {
    await client.query('BEGIN')
    var queries = queriesRaw.map(q => {
      return client.query(q[0], q[1])
    })

    for await (const oneResult of queries) {
      allResults.push(oneResult)
    }
    await client.query('COMMIT')
  } catch (err) {
    await client.query('ROLLBACK')
  } finally {
    client.release()
    return allResults
  }
}

And do transactions like this:

let results = await transaction([
            ['UPDATE readers SET cookies=5 WHERE id=$1;', [1]],
            ['INSERT INTO rewards (id) VALUES ($1);', [3]]
          ])

Transaction should do queries one at a time in array index sequence (so rollback to previos values will work correctly) and return in the same order (sometimes i need return values from some queries)

As i understand it starts already in map map function. In for await i just wait for results of it and second query may complete faster that previos.

So how can i fix this?

P.S. Maybe something like new Promise() instead map is the rigth way?

I'm sorry, just for understand, these are SQL that will run to your RDBMS? — Marco, May 21 '19 at 20:02
Both, because somethimes i return values from some of queries. — ZiiMakc, May 21 '19 at 20:06
@SLaks Promise.all does not await and i need it in transaction. — ZiiMakc, May 21 '19 at 20:11
i think issue might be that you are doing the `map` and effectively creating a number of `promise` objects. Why not `foreach` the raw array and `await` the result? — Bibberty, May 21 '19 at 20:11
@Bibberty so how then create array of promises for for await loop? — ZiiMakc, May 21 '19 at 20:12
@RTW: You should write `allResults = await Promise.all(queries)` — SLaks, May 21 '19 at 20:14
To run async code in sequence instead of in parallel, see https://blog.slaks.net/2015-06-10/advanced-promise-usage/#sequential-operations — SLaks, May 21 '19 at 20:15
@SLaks Promise.all start execution of all queries at the SAME time and for transaction rollback it's not usable. — ZiiMakc, May 21 '19 at 20:15
https://stackoverflow.com/questions/52697719/sequential-promises-using-async-await-in-vanilla-javascript — SLaks, May 21 '19 at 20:15
Can i just raise a red flag here? You are exposing and sending sql commands directly is a bad idea, can expose your database and make it easy to inject SQL. maybe at least you can encrypt the texts? — Marco, May 21 '19 at 20:16
@Marco how else can you send your sql commands to db :) $1 is protecting me for inject of untrusted values. — ZiiMakc, May 21 '19 at 20:17
@Marco: There is nothing wrong with exposing SQL in server-side code. — SLaks, May 21 '19 at 20:19

Bibberty · Answer 1 · 2019-05-21T20:27:09.047

2

Change this:

var queries = queriesRaw.map(q => {
  return client.query(q[0], q[1])
})

for await (const oneResult of queries) {
  allResults.push(oneResult)
}

To:

for(const q of rawQueries) {
   let result = await client.query(q[0], q[1]);
   allResults.push(result);
});

edited May 21 '19 at 20:27

answered May 21 '19 at 20:15

Bibberty

4,670
2
8
23

score 1 · Accepted Answer · edited May 21 '19 at 20:36

1

If i got you correctly, Just use a for loop with proper await, instead of a callback style loop.

So you can wait with the function to return unil everything is chronologically executed, With some thinking, you can easily add aa revoke() function or something..


...
export async function transaction(queriesRaw) {
  let allResults = []

  const client = await pool.connect()

  try {
    await client.query('BEGIN')

    for(var i = 0; i < queriesRaw.length;i++) {
        var res = await client.query(queriesRaw[i][0], queriesRaw[i][1])
        allResults.push(res)
    }

    await client.query('COMMIT')
  } catch (err) {
    await client.query('ROLLBACK') 

    // do you maybe wanna errors to results to?
    // allResults.push(err)
  } finally {
    client.release()
    return allResults
  }
}

Info,

Have a look at for example async module, or something similar. So you will not have to think about things like this.

edited May 21 '19 at 20:36

ZiiMakc

31,187
24
65
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answered May 21 '19 at 20:15

Silvan Bregy

2,544
1
8
21

1

I am, and just try it.. For explanation, You cannot use `await` within `forEach` because you pass a new function to it. So for having `await` in `forEach` you do this: `[fetch()].forEach(async res => console.log(await res))`. But then you'll have a problem because your code does not wait for the `forEach` to finish it's iterations. Read this: https://codeburst.io/javascript-async-await-with-foreach-b6ba62bbf404 – Silvan Bregy May 21 '19 at 20:23
1

thanks, for some reason this simple solution didn't get in my head as i was trying to make for await work :/ – ZiiMakc May 21 '19 at 20:26
1

That's why you should talk to other coders about your problems^^ – Silvan Bregy May 21 '19 at 20:27
It's solves my problem, but still question remains, how to make array of promises for new for await loop :) – ZiiMakc May 21 '19 at 20:28
I don't know either. I think you would have to use a generator: https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Statements/function* , And here's an example of await for loop :: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of – Silvan Bregy May 22 '19 at 08:44
The link pointing on `/function` should be `/function*`, with `*` – Silvan Bregy May 22 '19 at 08:46

Array of queries for `for await` loop for postgresql transaction helper

2 Answers2

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