mongo join between two collections

Question

I have two mongo collections:

1. defn

   "_id" : ObjectId("8570bebcb7db3"), 
   "fields" : [    {      
   "control" : {   "appearance" : "field-list" },  
   "children" : [ {       "bind" : {      "required" : "yes" },   ...
   

2. data

   "_id" : ObjectId("1570bf18a7db"),
   "defn" : ObjectId("8570bebcb7db3"),
   "data" : {
           "country" : "",
           "age" : 1,
           "age_unit" : "years", 
   },
   "label" : "type"
   

that are joined where defn._id =data.defn

How can I write a query to return data.label for defn._id? The query would be of the form db.defn.find({ data.label where defn._id= "X"})

Answer 1

If you're using MongoDB 3.2, the $lookup stage can perform the equivalent of a left outer join.

The documentation provides an example of using this operation here.

Example using your data:

db.defn.insert({
  "_id" : "123456", 
  "some_text" : "main document"
})

db.data.insert( { 
  "defn" : "123456",
  "label" : "data we want to access"
})

db.defn.aggregate( [ 
  { "$lookup" : {
     "from" : "data",
     "localField" : "_id", 
     "foreignField" : "defn",
     "as" : "defns"
    }
  } 
])

// Results:
// { 
//   "_id" : "123456", 
//   "some_text" : "main document", 
//   "defns" : [ { 
//      "_id" : ObjectId("57a2cbbbeb99ff285a1f0893"), 
//      "defn" : "123456", 
//      "label" : "data we want to access" 
//    } ] 
// }