Chaining a promise

Question

I check if a post exists in table. If it does not, I insert it. So I need to make two async calls. Can I do this flat with promises?

var insertOrUpdateBuilding = (callback)=>{
    // Check if building exists
    db('buildings')
      .where({externalId: buildingId})
      .then(function(rows){
        // Building exist, do nothing
        if(rows){
          callback ();
        }
        // Building does not exist. Insert it
        if(!rows){
          return db('buildings').insert({externalId: buildingId, name: req.body.name})
        }
      })
      .then(function(promise){

      })
      .catch(function(err){
        callback({message: 'Error looking up building', err: err})
      })
  };

I'm stuck. How do I proceed?

Generally you don't mix promises and callbacks. What's the database library you are using? — Tomalak, Nov 17 '16 at 10:10
http://stackoverflow.com/questions/12273437/jquery-deferred-object-with-nested-ajax-calls — Sunil B N, Nov 17 '16 at 10:11
@SunilBN The OP clearly is not using jQuery deferred objects. — Tomalak, Nov 17 '16 at 10:14
maybe it would be clearer to have `insertOrUpdateBuilding` return a `Promise`. — Davin Tryon, Nov 17 '16 at 10:14

georg · Accepted Answer · 2016-11-17T12:15:56.780

Promises are sticky, once you start using them, every involved function will have to accept and return promises as well. In your example, if db() is promised, then insertOrUpdateBuilding should return a promise too, that is, db(...) with some thens attached to it

var insertOrUpdateBuilding = () => db('buildings')
    .where({externalId: buildingId})
    .then(rows => rows ||
        db('buildings').insert({externalId: buildingId, name: req.body.name}))
    ;

and whoever calls insertOrUpdateBuilding should be promised as well:

  insertOrUpdateBuilding().then(okay).catch(error!)

Note that it generally doesn't make sense to catch errors in a lower-level function (except for logging/debugging purposes). Let the error propagate to the upper level where you can handle it in a sensible way, e.g. inform the user.

Niels Steenbeek · Answer 2 · 2016-11-17T10:20:52.020

0

Try using ES7 async/await.

var async insertOrUpdateBuilding = (callback) => {
    try {
        // Check if building exists
        let rows = await db('buildings')
          .where({externalId: buildingId});
        // Building exist, do nothing
        if (rows){
          callback();
        }
        // Building does not exist. Insert it
        if (!rows) {
          let row = await db('buildings').insert({
            externalId: buildingId, 
            name: req.body.name
          });
          callback();
        }
    } catch (e) {
        callback({message: 'Error looking up building', err: e});
    }
};

edited Nov 17 '16 at 10:20

answered Nov 17 '16 at 10:17

Niels Steenbeek

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Not sure this answers the question. The question was about promises, async/await might not be available. – Davin Tryon Nov 17 '16 at 10:20
@DavinTryon Thanks for the comment. I just added it. First skipped it to not mess up the code principle. – Niels Steenbeek Nov 17 '16 at 10:24

Chaining a promise

2 Answers2