Example:
> db.createCollection('testCol')

{ "ok" : 1 }

> db.testCol.insert({fish: {name: 'Arctic fish', desciption: 'I like cold water!'}})

And now we a going to find our recond.
> db.testCol.find({fish: {name: 'Arctic fish', desciption: 'I like cold water!'}})
{ "_id" : ObjectId("503f37b12f2ef2c8d4b9a430"), "fish" :

{ "name" : "Arctic fish", "desciption" : "I like cold water!" }

}

But if we define fields in other order, mongo can't find our record.
> db.testCol.find({fish:

{desciption: 'I like cold water!', name: 'Arctic fish'}

)