Unable to resolve promise chain

Question

I am trying to write a Sequelize migration script win which I am trying to update my database but it is having many asynchronous operations (database queries and then updating database with particular id)

Here is my code

return db.organizationEntries
      .findAll()
      .then((entries) => {
        return entries.forEach(entry => {
          console.log(entry);
          db.organizationEntries
            .findAll({
              attributes: [
                [
                  db.sequelize.fn(
                    'MAX',
                    db.sequelize.col('organizationEntries.serial_number')
                  ),
                  'maximum_serial_no'
                ]
              ],
              where: {
                organizationId: entry.organizationId
              }
            })
            .then(orgEntry => {
              console.log(orgEntry[0].dataValues.maximum_serial_no);
              let data = { serialNumber: orgEntry[0].dataValues.maximum_serial_no + 1 };
              console.log(data)

              //problem 
              db.organizationEntries.update(data, {
                where: {
                  id: entry.id
                }
              })
                .then((result) => {
                  console.log(result);

                })
            });
          // promises.push(promise);
        });
        // return Promise.all(promises);
      })

Actually what I am trying to do is I am trying to take the list of all orgEntries from the database and then I am finding maximum serial number for that organization_id and then updating that particular orgEntry and like this all these operations in a loop

Now the problem is coming all the things are going in order but after finding max_serial_no it is not updating the database and I am not able to resolve what I should do to make that asynchronous call work in this order

The `return entries.forEach` is not going to do what you want - `forEach` returns undefined. Is your `// promises.push(promise);` relevant? — CertainPerformance, Apr 25 '18 at 08:25
@CertainPerformance I have tried to put this update in Promise.all() but I want to occur all things in order as I have mentioned else wrong max_serial_no will come — Vikas, Apr 25 '18 at 08:28
Instead of `.forEach` do `return Promise.all(entries.map(fn => ...)` (and return the promise from it) — Benjamin Gruenbaum, Apr 25 '18 at 08:42
@Vikas I'm gonna delete my answer because it feels out of context, please elaborate what did you mean by update single row at a time? does the order of the entries read from db is the one that matters , or just updating one by one - which ever finishes first the 'Max' query? — Nirit Levi, Apr 25 '18 at 09:34
@NiritLevi Don't delete it . It was my mistake it is working fine now . U really have a good knowledge of Promises as I was unaware of this till — Vikas, Apr 25 '18 at 09:47

Carmelo Catalfamo · Accepted Answer · 2018-04-25T09:33:00.260

I think you can solve this in two ways:

Simultaneously Promises

In a following code I removed forEach in favor of Promise.all() and map()

The map() method create (and return) a new array with the results of calling a provided function on every element in the calling array.

Example:

let numbers = [1, 2, 3]
let doubledNumbers = numbers.map(n => n * 2)
// doubledNumbers [2, 4, 6]

The Promise.all() method take an array of Promises as argument and returns a single Promise that will be resolved when all promises will be resolved or rejected if one promise failed

Example:

let promise1 = findUserById(5)
let promise2 = findUserFriends(5)
Promise.all([promise1, promise2])
.then(values => {
  // values: [user object, list of user friends]
})

Result:

db.organizationEntries.findAll()
.then(entries => {
  return Promise.all(entries.map(entry => {
    console.log(entry)

    return db.organizationEntries.findAll({
      where: {
        organizationId: entry.organizationId
      },
      attributes: [
        [
          db.sequelize.fn('MAX', db.sequelize.col('organizationEntries.serial_number')),
          'maximum_serial_no'
        ]
      ]
    })
    .then(orgEntry => {
      console.log(orgEntry[0].dataValues.maximum_serial_no)
      let data = { serialNumber: orgEntry[0].dataValues.maximum_serial_no + 1 }
      console.log(data)
      return db.organizationEntries.update(data, { where: { id: entry.id } })
    })
  }))
})
.then(result => {
  // result: Array of updated organizationEntries
  console.log(result)
})

Step by step Promises with reduce() method

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. (from MDN web docs)

Example:

let items = [{ name: 'pencil', price: 2 }, { name: 'book', price: 10 }]
let total = items.reduce((total, item) => total += item.price, 0)
// total: 12

Result:

db.organizationEntries.findAll()
.then(entries => {
  return entries.reduce((previousPromise, entry) => {
    console.log(entry)

    return previousPromise
    .then(_ => {
      return db.organizationEntries.findAll({
        where: {
          organizationId: entry.organizationId
        },
        attributes: [
          [
            db.sequelize.fn('MAX', db.sequelize.col('organizationEntries.serial_number')),
            'maximum_serial_no'
          ]
        ]
      })
    })
    .then(orgEntry => {
      console.log(orgEntry[0].dataValues.maximum_serial_no)
      let data = { serialNumber: orgEntry[0].dataValues.maximum_serial_no + 1 }
      console.log(data)
      return db.organizationEntries.update(data, { where: { id: entry.id } })
    })
    .then(updatedEntry => {
      console.log(updatedEntry)
    })
  }, Promise.resolve())
})
.then(result => {
  // result: Last updated organization entry
  console.log('finished')
})

Hey Carmelo, thanks for your answer - can you elaborate on why this works? (Why are we `Promise.all`ing `entries.map`?) — Benjamin Gruenbaum, Apr 25 '18 at 09:01
Promise.all() will do all the updates simultaneously but I want one by one — Vikas, Apr 25 '18 at 09:19
@Vikas oh ok! Just replace a `map()` method with a `reduce()`. I'll write example soon. — Carmelo Catalfamo, Apr 25 '18 at 09:20
@NiritLevi that was a rhetorical question in an attempt to ask Carmelo to explain this in the answer, but thanks :) — Benjamin Gruenbaum, Apr 25 '18 at 09:27

Unable to resolve promise chain

1 Answers1