MongoDB - Fetch an object from deep subdocuments - mongodb

Question

How to query an object from an Array inside an Array, and get it as a top-level object? For example, consider the following record.
{
"subjects": [
{
"name": "English",
"teachers": [
{
"name": "Mark" /* Trying to get this object*/
},
{
"name": "John"
}
]
}
]
}
I am trying to get the following object out as the top-level object.
{
"name": "Mark"
}

Answer 1

You need to use the aggregation framework to do exactly what you're asking for.
Here I entered the document you gave into collection: foo.
> db.foo.find().pretty()
{
"_id" : ObjectId("57ceed3d31484d5b491eaae9"),
"subjects" : [
{
"name" : "English",
"teachers" : [
{
"name" : "Mark"
},
{
"name" : "John"
}
]
}
]
}
Using $unwind to unravel our array we then enter our first stage of the aggregation pipeline:
> db.foo.aggregate([
... {$unwind: "$subjects"}
... ]).pretty()
{
"_id" : ObjectId("57ceed3d31484d5b491eaae9"),
"subjects" : {
"name" : "English",
"teachers" : [
{
"name" : "Mark"
},
{
"name" : "John"
}
]
}
}
Subjects was an array of length 1 so the only difference here is one less set of [] array brackets.
We need to unwind again.
> db.foo.aggregate([
... {$unwind: "$subjects"},
... {$unwind: "$subjects.teachers"}
... ]).pretty()
{
"_id" : ObjectId("57ceed3d31484d5b491eaae9"),
"subjects" : {
"name" : "English",
"teachers" : {
"name" : "Mark"
}
}
}
{
"_id" : ObjectId("57ceed3d31484d5b491eaae9"),
"subjects" : {
"name" : "English",
"teachers" : {
"name" : "John"
}
}
}
Now we turned our array of length '2' into two separate documents. The first one with subjects.teachers.name = Mark and the second with subjects.teachers.name = John.
We only want to return the case where name = Mark so we need to add a $match stage to our pipeline.
> db.foo.aggregate([
... {$unwind: "$subjects"},
... {$unwind: "$subjects.teachers"},
... {$match: {"subjects.teachers.name": "Mark"}}
... ]).pretty()
{
"_id" : ObjectId("57ceed3d31484d5b491eaae9"),
"subjects" : {
"name" : "English",
"teachers" : {
"name" : "Mark"
}
}
}
Ok! Now we are only matching on the case where name: Mark.
Let's add a $project case to shape our input how we want.
> db.foo.aggregate([
... {$unwind: "$subjects"},
... {$unwind: "$subjects.teachers"},
... {$match: {"subjects.teachers.name": "Mark"}},
... {$project: {"name": "$subjects.teachers.name", "_id": 0}}
... ]).pretty()
{ "name" : "Mark" }