angular chaining arrays of promises

Question

I am building a website over a database of music tracks. The database is as follows :

• music table contains musicid and title
• musicrights table contains musicid and memberid
• members table contains memberid and memberinfo.

I'm trying to build an array of objects in my database service, which each entry represents a track containing its rightholders (contains information aubout one rightholder but not his name) and their member info (contains name etc). The backend is sailsjs and the code is as follows :

angular.module("myapp").service("database", ["$q", "$http", function($q, $http) {

  var database = {};

  function getHolderMember(rightHolder) {
    return ($http.get("/api/members?where=" + JSON.stringify({
      memberid: rightHolder.memberid
    })).then(function (res) {
        rightHolder.member = res.data[0];
        return (rightHolder);
    }));
  }

  function getRightHolders(doc) {
    return ($http.get("/api/musicrights?where=" + JSON.stringify({
      musicid: doc.musicid
    })).then(function(res) {
      // array of promises :
      // each rightholder of a document has to solve member info
      var rightHolders = [];
      for (var i in res.data) {
        var rightHolder = {
          member: res.data[i].memberid,
          type: res.data[i].membertype,
          rights: res.data[i].memberrights
        };
        rightHolders.push(getHolderMember(rightHolder));
      }
      return ($q.all(rightHolders));
    }).then(function(rightHolders) {
      // expected array of one or two rightholders,
      // enriched with member information

      // actually returns array of one or two arrays of 30 members
      // without rightholder info
      console.log(rightHolders);
      doc.rightHolders = rightHolders;
      return (doc);
    }));
  }

  database.music = function(q) {
    return ($http.get("/api/music?where=" + JSON.stringify({
      or: [{
        title: {
          contains: q
        }
      }, {
        subtitle: {
          contains: q
        }
      }]
    })).then(function(res) {
      // array of 30 promises :
      // each one of 30 documents has to resolve its rightholders
      var documents = [];
      for (var i in res.data) {
        documents.push(getRightHolders(res.data[i]));
      }
      return ($q.all(documents));
    }));
  }

  return (database);
}]);

The first array of promises seems to work as expected, but not the second one in getRightHolders. What is strange is that this function returns an array of one or two promises, which are rightHolders waiting for their memberinfo. But in the callback where I console.log the response, i get an array of one or two (as per the number of pushed promises) but this array's elements are arrays of 30 memberinfo instead of one memberinfo. I don't understand how this $q.all() call gets mixed with the previous-level $q.all.

The data structure is roughly like this

• documents [ ] ($http => 30 responses)
  • music.musicid
  • music.rightHolders [ ] ($http => 1, 2, 3 responses)
    • rightholder.rights
    • rightholder.member ($http => 1 response)
      • member.memberinfo

Any help appreciated. Thank you !

UPDATE : Thank you for your answer, it worked like a charm. Here's the updated code, with also the migrate service which formats data differently (there is some database migration going on). I kept it out of the first example but your answer gave me this neat syntax.

angular.module("myApp").service("database", ["$q", "$http", "migrate", function($q, $http, migrate) {

        var database = {};

        function getHolderMember(rightHolder) {
          return ($http.get("/api/members?where=" + JSON.stringify({
            memberID: rightHolder.member
          })).then(function(res) {
            return (migrate.member(res.data[0]));
          }).then(function(member) {
            rightHolder.member = member;
            return (rightHolder);
          }));
        }

        function getRightHolders(doc) {
          return ($http.get("/api/rightHolders?where=" + JSON.stringify({
            musicID: doc.musicID
          })).then(function(res) {
            return (
              $q.all(res.data
                .map(migrate.rightHolder)
                .map(getHolderMember)
              )
            );
          }).then(function(rightHolders) {
            doc.rightHolders = rightHolders;
            return (doc);
          }));
        }

        database.music = function(q) {
          return ($http.get("/api/music?where=" + JSON.stringify({
            or: [{
                title: {
                  contains: q
                }
              },
              {
                subtitle: {
                  contains: q
                }
              }
            ]
          })).then(function(res) {
            return (
              $q.all(res.data
                .map(migrate.music)
                .map(getRightHolders)
              )
            );
          }));
        }

        return (database);
      }

Answer 1

I'm not quite sure how you're getting the result you describe, but your logic is more convoluted than it needs to be and I think this might be leading to the issues you're seeing. You're giving the getRightsHolders function the responsibility of returning the document and based on your comment above, it sounds like you previously had the getHolderMember() function doing something similar and then stopped doing that.

We can clean this up by having each function be responsible for the entities it's handling and by using .map() instead of for (please don't use for..in with arrays).

Please give this a try:

angular
  .module("myapp")
  .service("database", ["$q", "$http", function($q, $http) {
    var database = {};

    function getHolderMember(memberId) {
      var query = JSON.stringify({ memberid: memberid });
      return $http.get("/api/members?where=" + query)
          .then(function (res) {
              return res.data[0];
          });
    }

    function populateRightsHolderWithMember(rightsHolder) {
       return getHolderMember(rightsHolder.memberid)
         .then(function (member) {
           rightsHolder.member = member;
           return rightsHolder;
         });
    }

    function getRightHolders(doc) {
      var query = JSON.stringify({ musicid: doc.musicid });

      return $http.get("/api/musicrights?where=" + query)
        .then(function(res) {
          return $q.all(res.data.map(populateRightsHolderWithMember));
        });
    }

    function populateDocumentWithRightsHolders(document) {
      return getRightsHolders(document)
        .then(function(rightsHolders) {
          document.rightsHolders = rightsHolders;
          return document;
        });
    }

    database.music = function(q) {
      return $http.get("/api/music?where=" + JSON.stringify({
        or: [{
          title: {
            contains: q
          }
        }, {
          subtitle: {
            contains: q
          }
        }]
      })).then(function(res) {
        return $q.all(res.data.map(populateDocumentWithRightsHolders));
      });
    }

    return (database);
  }]);