CODEWITHSUNDEEP
mongodb
Anders Östman
Anders Östman

Reputation: 3832

Find a set of documents in collection A, based on an array from collection B

The following data needs to be stored in MongoDb:

A collection of persons (approximately 100-2000) together with their relevant attributes. Another collection of queues (approximately 5-50).

Information about the relationsship between persons and queues. Each person can stand in line in several queues, and each queue can hold several persons. The order of the persons waiting in a queue is important.

Currently this is what i have in mind:

Persons:

{
  _id: ObjectId("507c35dd8fada716c89d0001"),
  first_name: 'john',
  email: '[email protected]'
  id_number: 8101011234,
  ... 
},

Queues:

{
  _id: ObjectId("507c35dd8fada716c89d0011"),
  title: 'A title for this queue',
  people_waiting: [
    ObjectId("507c35dd8fada716c89d0001"),
    ObjectId("507c35dd8fada716c89d0002"), 
    ObjectId("507c35dd8fada716c89d0003"), 
    ...
  ]
},

In a web page, I want to list (in order) all persons standing in a certain queue. I'm thinking that I first need to query the 'people_waiting' array from the 'Queues' collection. And then loop trough this array and for each item query it from the 'Persons' collection.

But there seems to be a lot of queries to generate this list, and i wonder if there is a smarter way to write/combine queries than the way described above.

Upvotes: 0

Views: 67

Answers (1)

JohnnyHK
JohnnyHK

Reputation: 312129

You can only query one collection at a time in MongoDB, so it does take two queries. But you can use $in instead of looping through array and querying each person individually.

In the shell:

queue = db.Queues.findOne({_id: idOfQueue});
peopleWaiting = db.Persions.find({_id: {$in: queue.people_waiting}}).toArray();

But peopleWaiting will not be sorted by the order of the ids in the queue and there's no support for doing that in a MongoDB query. So you'd have to reorder peopleWaiting in your code to match the order in queue.people_waiting.

Upvotes: 1

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