How to query nested objects?

Question

I have a problem when querying mongoDB with nested objects notation:

db.messages.find( { headers : { From: \"reservations@marriott.com\" } } ).count()
0
db.m         


        

Answer 1

The two query mechanism work in different ways, as suggested in the docs at the section Subdocuments:

When the field holds an embedded document (i.e, subdocument), you can either specify the entire subdocument as the value of a field, or “reach into” the subdocument using dot notation, to specify values for individual fields in the subdocument:

Equality matches within subdocuments select documents if the subdocument matches exactly the specified subdocument, including the field order.

---

In the following example, the query matches all documents where the value of the field producer is a subdocument that contains only the field company with the value 'ABC123' and the field address with the value '123 Street', in the exact order:

db.inventory.find( {
    producer: {
        company: 'ABC123',
        address: '123 Street'
    }
});

Answer 2

Since there is a lot of confusion about queries MongoDB collection with sub-documents, I thought its worth to explain the above answers with examples:

First I have inserted only two objects in the collection namely: message as:

> db.messages.find().pretty()
{
    "_id" : ObjectId("5cce8e417d2e7b3fe9c93c32"),
    "headers" : {
        "From" : "reservations@marriott.com"
    }
}
{
    "_id" : ObjectId("5cce8eb97d2e7b3fe9c93c33"),
    "headers" : {
        "From" : "reservations@marriott.com",
        "To" : "kprasad.iitd@gmail.com"
    }
}
>

So what is the result of query: db.messages.find({headers: {From: "reservations@marriott.com"} }).count()

It should be one because these queries for documents where headers equal to the object {From: "reservations@marriott.com"}, only i.e. contains no other fields or we should specify the entire sub-document as the value of a field.

So as per the answer from @Edmondo1984

Equality matches within sub-documents select documents if the subdocument matches exactly the specified sub-document, including the field order.

From the above statements, what is the below query result should be?

> db.messages.find({headers: {To: "kprasad.iitd@gmail.com", From: "reservations@marriott.com"}  }).count()
0

And what if we will change the order of From and To i.e same as sub-documents of second documents?

> db.messages.find({headers: {From: "reservations@marriott.com", To: "kprasad.iitd@gmail.com"}  }).count()
1

so, it matches exactly the specified sub-document, including the field order.

For using dot operator, I think it is very clear for every one. Let's see the result of below query:

> db.messages.find( { 'headers.From': "reservations@marriott.com" }  ).count()
2

I hope these explanations with the above example will make someone more clarity on find query with sub-documents.

Answer 3

db.messages.find( { headers : { From: "reservations@marriott.com" } } )

This queries for documents where headers equals { From: ... }, i.e. contains no other fields.

---

db.messages.find( { 'headers.From': "reservations@marriott.com" } )

This only looks at the headers.From field, not affected by other fields contained in, or missing from, headers.

---

Dot-notation docs