Retrieve only some fields from sub-document

Question

(I am an extreme newbie with extremely simple question):

If my database (from the documentation samples) consists of:

{ "item" : "journal", "qty" : 25, "size" : { "h" : 14, "w" : 21, "uom" : "cm" }, "status" : "A" }
{ "item" : "notebook", "qty" : 50, "size" : { "h" : 8.5, "w" : 11, "uom" : "in" }, "status" : "A" }
{ "item" : "paper", "qty" : 100, "size" : { "h" : 8.5, "w" : 11, "uom" : "in" }, "status" : "D" }
{ "item" : "planner", "qty" : 75, "size" : { "h" : 22.85, "w" : 30, "uom" : "cm" }, "status" : "D" }
{ "item" : "postcard", "qty" : 45, "size" : { "h" : 10, "w" : 15.25, "uom" : "cm" }, "status" : "A" }

I issue this command:

db.inventory.find( { item: /^j/ }, { size: 1 } )

I get back this document:

{
    "_id" : ObjectId("59448e8544c2d6b6ced4ac66"),
    "size" : {
        "h" : 14.0,
        "w" : 21.0,
        "uom" : "cm"
    }
}

What I want to do is, only get back "h". If I try this command:

db.inventory.find( { item: /^j/ }, { size.h: 1 } )

I get "Unexpected token ."

How can I cause only the "h" field of array "size" to return?

Thanks!

PS, I tried the example and answer in how to retrieve partial objects from object array in a field in mongodb, but the result was "Script executed successfully, but there are no results to show."

Because it's not an array. And the very specific error message should have pointed you to the "dot" as the clear source of the problem. Look at "Dot Notation" in the core documentation and the very clear example "<embedded document>.<field>" which very clearly shows the "quotes" "" — Neil Lunn
– Neil Lunn, Commented Jun 17, 2017 at 9:02

David Gourde · Accepted Answer · 2018-03-08 18:21:18Z

2

You have to enclose the key with quotes so it's correct syntax. A key can only contain $, _, and alphanumeric characters. If the key is invalid inside the object literal, just enclose the key with quotes to bypass this:

db.inventory.find( { item: /^j/ }, { 'size.h': 1 } )

edited Mar 8, 2018 at 18:21

David Gourde

3,9442 gold badges36 silver badges75 bronze badges

answered Jun 17, 2017 at 2:58

Andrew Li

58.1k14 gold badges135 silver badges150 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

hardy · Accepted Answer · 2017-06-17 04:58:18Z

0

//use when you want `_id,size` (As object) with its only property `h` in 
//$project,

db.inventory.aggregate([{$match:{  item: /^j/  } },{ $project:{ 
"size.h":"$size.h" } }])

//use when you want `_id,size` size ase whole object  
//$project,

db.inventory.aggregate([{$match:{  item: /^j/  } },{ $project:{ "size":1 } 
}])

You can use Aggregation which can help you to get data in any form .
It includes Pipes.
It accepts array

Note :- To ignore any property just change 1 to 0 in $project properties

Thanks

answered Jun 17, 2017 at 4:58

hardy

9013 gold badges14 silver badges31 bronze badges

1 Comment

Knom Over a year ago

The other answer is exactly what I need for now. However, I really appreciate your answer and will study it because I am sure I will learn something I really need to know. Thanks very much!

Collectives™ on Stack Overflow

Retrieve only some fields from sub-document

2 Answers 2

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related