Find document in subarray in mongodb

Question

I have many collections such this

{
"_id" : 383566,
"horses" : {
    "609075" : {
        "_id" : 609075,
        "name" : "Aghawonan"
    },
    "634478" : {
        "_id" : 634478,
        "name" : "French Thyne"
    },
    "595535" : {
        "_id" : 595535,
        "name" : "Doctor Hanley"
    },
    "593718" : {
        "_id" : 593718,
        "name" : "Ballyhill Boy"
    },
    "578089" : {
        "_id" : 578089,
        "name" : "Oh Alabama"
    },
    "632597" : {
        "_id" : 632597,
        "name" : "Bowling Alone"
    },
    "525424" : {
        "_id" : 525424,
        "name" : "Tally-Ho Duke"
    }
}

I need to find all the collections in the array where Horses _id have match

My version, but it does not work

db.cards.find({'horses': {$elemMatch: {_id:525424}}})

mnemosyn · Accepted Answer · 2014-06-02 16:20:56Z

1

in your example, horses is not an array, but an object. This data structure is a bit odd and redundant because the id is both part of the 'name' (or path if you will) and the actual document data. In your data structure, you could refer to a particular horse by horses.609075.name, but that is usually not very helpful.

Your query would work if horses were an array like so:

{
  "_id" : 383566,
  "horses" : [
    {
        "_id" : 609075,
        "name" : "Aghawonan"
    },
    {
        "_id" : 634478,
        "name" : "French Thyne"
    },
    // ...
   ]
 }

Since you're only querying for a single property, the $elemMatch isn't required. You could simply query like this:

db.cards.find({"horses._id" : 609075});

Please note that you'll receive the entire card as a result, not just a part of the array.

As a side note, why are the ids of the horses called _id? The name _id is a special field only at the root level. Embedded documents don't have any special fields.

answered Jun 2, 2014 at 16:20

mnemosyn

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7 Comments

user3603475 Over a year ago

Sorry, horses - Object

mnemosyn Over a year ago

That comment doesn't seem to make any sense? If you're bound to model horses as an object, you're probably doing something wrong.

user3603475 Over a year ago

I already knew it possible to somehow change the field type Horses with an array object to 139,000 records

user3603475 Over a year ago

db.cards.find({"horses._id" : 609075}); - not working

mnemosyn Over a year ago

Of course not, unless you change your model to an array as I pointed out in my answer. You should review the terms used in MongoDB (collections, array and documents) and change your data structure.

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