Find & return first matching subdocument from array (Meteor / Mongo)

Question

how do I find and return the first subdocument in the 'tasks' array that matches completed: true?

using findOne returns the entire document.. is there another function for returning a subdocument?

{
  title: 'awebsite.com'
  company: 'a company'
  companyID: Random.id()
  category: 'website'
  starred: false
  timeline: {
    tasks: [
      {
        name: 'task1'
        completed: true
        todos: [
          {todo: 'something', completed: false, todoID: Random.id()}
          {todo: 'something', completed: false, todoID: Random.id()}
          {todo: 'something', completed: false, todoID: Random.id()}
        ]
      }
      {
        name: 'task2'
        completed: false
        todos: [
          {todo: 'something', completed: false, todoID: Random.id()}
          {todo: 'something', completed: false, todoID: Random.id()}
          {todo: 'something', completed: false, todoID: Random.id()}
        ]
      }
    ]
  }
}

Answer 1

You can do this by aggregation where you can take advantage of an index and limit with the $match pipeline. Use the $unwind operator to deconstruct your tasks array into a stream of documents that can then be matched. since you would want to return only "the first subdocument in the 'tasks' array that matches completed: true", you could use the $limit operator in the last pipeline stage to return only one subdocument:

db.collection.aggregate([
  { 
      $match: {
          "timeline.tasks.completed": true
      }
  },
  { 
      $unwind: "$timeline.tasks" 
  },
  { 
      $match: {
          "timeline.tasks.completed": true
      }
  },
  { 
      $group: {
           _id: {
                "tasks": "$timeline.tasks"
           }
      }
  },
  {
      $project: {
          _id: 0,
          tasks: "$_id.tasks"
      }
  },
  {
      $limit: 1
  }
])

Results in:

{
    "result" : [ 
        {
            "tasks" : {
                "name" : "task1",
                "completed" : true,
                "todos" : [ 
                    {
                        "todo" : "something",
                        "completed" : false,
                        "todoID" : "jfoe84jn"
                    }, 
                    {
                        "todo" : "something",
                        "completed" : false,
                        "todoID" : "yr934hjs"
                    }, 
                    {
                        "todo" : "something",
                        "completed" : false,
                        "todoID" : "84hdkl0t"
                    }
                ]
            }
        }
    ],
    "ok" : 1
}