Promises don't behave as expected in migrations

(This may be a duplicate of #312)

A migration file like so:

exports.up = function(knex, Promise) {
  var first = knex.schema.createTable('first', function(table) {
    table.increments('id');
    table.string('name');
  });

  var second = first.then(function() {
    return knex.schema.createTable('second', function(table) {
      table.increments('id');
      table.string('name');
    });
  });

  return Promise.all([first, second]);
};

exports.down = function(knex, Promise) {

};

gives the following output:

{ __cid: '__cid1',
  sql: 'create table "first" ("id" serial primary key, "name" varchar(255))',
  bindings: [] }
{ __cid: '__cid2',
  sql: 'create table "first" ("id" serial primary key, "name" varchar(255))',
  bindings: [] }
{ __cid: '__cid3',
  sql: 'create table "second" ("id" serial primary key, "name" varchar(255))',
  bindings: [] }
error: duplicate key value violates unique constraint "pg_type_typname_nsp_index"
    at Connection.parseE (/home/sohum/node_modules/pg/lib/connection.js:526:11)
    at Connection.parseMessage (/home/sohum/node_modules/pg/lib/connection.js:356:17)
    at Socket.<anonymous> (/home/sohum/node_modules/pg/lib/connection.js:105:22)
    at Socket.EventEmitter.emit (events.js:95:17)
    at Socket.<anonymous> (_stream_readable.js:745:14)
    at Socket.EventEmitter.emit (events.js:92:17)
    at emitReadable_ (_stream_readable.js:407:10)
    at emitReadable (_stream_readable.js:403:5)
    at readableAddChunk (_stream_readable.js:165:9)
    at Socket.Readable.push (_stream_readable.js:127:10)

[tgriesser]

So the confusion here is that the createTable method does not return a promise, but rather it returns a SchemaBuilder object, which is a “thenable”, i.e. calling .then on the object will return a valid A+ promise, but the object its self is not a promise.

This was done specifically to allow you to use the syntax:

  return knex.schema.createTable('first', function(table) {
    table.increments('id');
    table.string('name');
  })
  .createTable('second', function(table) {
      table.increments('id');
      table.string('name');
  }).then(function() {
    // all done
  });

which should run the migrations as expected on the same connection in sequence.

Also, there is no need for Promise.all here, as this would accomplish the same:

exports.up = function(knex, Promise) {
  return knex.schema.createTable('first', function(table) {
    table.increments('id');
    table.string('name');
  }).then(function() {
    return knex.schema.createTable('second', function(table) {
      table.increments('id');
      table.string('name');
    });
  });
};

though if you wanted, you could do:

exports.up = function(knex, Promise) {
  var first = knex.schema.createTable('first', function(table) {
    table.increments('id');
    table.string('name');
  }).then(); // coerce thenable to a promise

  var second = first.then(function() {
    return knex.schema.createTable('second', function(table) {
      table.increments('id');
      table.string('name');
    });
  });

  return Promise.all([first, second]);
};

and things would work as defined by the spec.

[SohumB]

Yea, we have more complicated dependency graphs than that; this was just a minimal test case! Thanks for the help.

@johanneslumpe

sohum@diurnal ~ % cat test.js
var Promise = require('bluebird');

function promises() {
  var first = Promise.resolve('wee');
  var second = first.then(function() {
    console.log('delayin');
    return Promise.delay(1000);
  }).then(function() {
    console.log('done!');
  });
  return first;
}

promises().then(function() { console.log('yep, first is finished'); });

sohum@diurnal ~ % node test.js
delayin
yep, first is finished
done!