Задача: в БД с данными о видах и конкретных особях животных найти тех, кто питается одинаковым кормом. Исходные данные:

<code javascript>db.animals.insert({animal_type:"l1", individuals: [ {name: "", text: "", feed: [1, 2] }, {name: "", text: "", feed: [3,4] }]})
db.animals.insert({animal_type:"l2", individuals: [ {name: "", text: "", feed: [1, 2] }, {name: "", text: "", feed: [2] }]})
db.animals.insert({animal_type:"l3", individuals: [ {name: "", text: "", feed: [1, 2] }, {name: "", text: "", feed: [3] }]})</code>

Решение:

**db.animals.find({'individuals.feed': 3})**

Вывод:

<code javascript>
{
  "_id" : ObjectId("5834832075ec6ecba9a62c11"), 
  "animal_type" : "l1", 
  "individuals" : [ 
    { "name" : "", "text" : "", "feed" : [ 1, 2 ] }, 
    { "name" : "", "text" : "", "feed" : [ 3, 4 ] } 
  ] 
}
{ 
  "_id" : ObjectId("5834833975ec6ecba9a62c13"), 
  "animal_type" : "l3", 
  "individuals" : [ 
    { "name" : "", "text" : "", "feed" : [ 1, 2 ] }, 
    { "name" : "", "text" : "", "feed" : [ 3 ] } 
  ] 
}
</code>