Project Fields to Return from Query

https://www.mongodb.com/docs/manual/tutorial/project-fields-from-query-results/

 

Project Fields to Return from Query

 

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By default, queries in MongoDB return all fields in matching documents. To limit the amount of data that MongoDB sends to applications, you can include a projection document to specify or restrict fields to return.

 

This page provides examples of query operations with projection using the db.collection.find() method in mongosh.

The examples on this page use the inventory collection. Connect to a test database in your MongoDB instance then create the inventory collection:

 
 
 
 
 
 
 
 
 
 
 
 
 
db.inventory.insertMany( [
{ item: "journal", status: "A", size: { h: 14, w: 21, uom: "cm" }, instock: [ { warehouse: "A", qty: 5 } ] },
{ item: "notebook", status: "A", size: { h: 8.5, w: 11, uom: "in" }, instock: [ { warehouse: "C", qty: 5 } ] },
{ item: "paper", status: "D", size: { h: 8.5, w: 11, uom: "in" }, instock: [ { warehouse: "A", qty: 60 } ] },
{ item: "planner", status: "D", size: { h: 22.85, w: 30, uom: "cm" }, instock: [ { warehouse: "A", qty: 40 } ] },
{ item: "postcard", status: "A", size: { h: 10, w: 15.25, uom: "cm" }, instock: [ { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ] }
]);
 
 
 
 
 
 
 
 
 
 
 
 

Return All Fields in Matching Documents

 
 

If you do not specify a projection document, the db.collection.find() method returns all fields in the matching documents.

 
 
 
 
 
 
 
 
 
 

The following example returns all fields from all documents in the inventory collection where the status equals "A":

 
db.inventory.find( { status: "A" } )
 
 
 
 
 
 
 
 
 
 
 
 

The operation corresponds to the following SQL statement:

SELECT * from inventory WHERE status = "A"
 

Return the Specified Fields and the _id Field Only

 

A projection can explicitly include several fields by setting the <field> to 1 in the projection document. The following operation returns all documents that match the query. In the result set, only the item, status and, by default, the _id fields return in the matching documents.

 
db.inventory.find( { status: "A" }, { item: 1, status: 1 } )
 
 
 
 
 
 
 
 
 
 
 
 

The operation corresponds to the following SQL statement:

SELECT _id, item, status from inventory WHERE status = "A"
 

Suppress _id Field

 

You can remove the _id field from the results by setting it to 0 in the projection, as in the following example:

 
db.inventory.find( { status: "A" }, { item: 1, status: 1, _id: 0 } )
 
 
 
 
 
 
 
 
 
 
 
 

The operation corresponds to the following SQL statement:

SELECT item, status from inventory WHERE status = "A"
 

NOTE

With the exception of the _id field, you cannot combine inclusion and exclusion statements in projection documents.

Return All But the Excluded Fields

 

Instead of listing the fields to return in the matching document, you can use a projection to exclude specific fields. The following example which returns all fields except for the status and the instock fields in the matching documents:

 
db.inventory.find( { status: "A" }, { status: 0, instock: 0 } )
 
 
 
 
 
 
 
 
 
 
 
 

NOTE

With the exception of the _id field, you cannot combine inclusion and exclusion statements in projection documents.

Return Specific Fields in Embedded Documents

 

You can return specific fields in an embedded document. Use the dot notation to refer to the embedded field and set to 1 in the projection document.

The following example returns:

  • The _id field (returned by default),

  • The item field,

  • The status field,

  • The uom field in the size document.

The uom field remains embedded in the size document.

 
db.inventory.find(
{ status: "A" },
{ item: 1, status: 1, "size.uom": 1 }
)
 
 
 
 
 
 
 
 
 
 
 
 

Starting in MongoDB 4.4, you can also specify embedded fields using the nested form, e.g. { item: 1, status: 1, size: { uom: 1 } }.

Suppress Specific Fields in Embedded Documents

 

You can suppress specific fields in an embedded document. Use the dot notation to refer to the embedded field in the projection document and set to 0.

The following example specifies a projection to exclude the uom field inside the size document. All other fields are returned in the matching documents:

 
db.inventory.find(
{ status: "A" },
{ "size.uom": 0 }
)
 
 
 
 
 
 
 
 
 
 
 
 

Starting in MongoDB 4.4, you can also specify embedded fields using the nested form, e.g. { size: { uom: 0 } }.

Projection on Embedded Documents in an Array

 

Use dot notation to project specific fields inside documents embedded in an array.

The following example specifies a projection to return:

  • The _id field (returned by default),

  • The item field,

  • The status field,

  • The qty field in the documents embedded in the instock array.

 
db.inventory.find( { status: "A" }, { item: 1, status: 1, "instock.qty": 1 } )
 
 
 
 
 
 
 
 
 
 
 
 

Project Specific Array Elements in the Returned Array

 
 

For fields that contain arrays, MongoDB provides the following projection operators for manipulating arrays: $elemMatch, $slice, and $.

The following example uses the $slice projection operator to return the last element in the instock array:

 
db.inventory.find( { status: "A" }, { item: 1, status: 1, instock: { $slice: -1 } } )
 
 
 
 
 
 
 
 
 
 
 
 

$elemMatch, $slice, and $ are the only way to project specific elements to include in the returned array. For instance, you cannot project specific array elements using the array index; e.g. { "instock.0": 1 } projection will not project the array with the first element.

 
 
 
 
 
 
 
 
 
 
 

Additional Considerations

 

Starting in MongoDB 4.4, MongoDB enforces additional restrictions with regards to projections. See Projection Restrictions for details.

TIP

posted @ 2023-08-13 02:05  功夫 熊猫  阅读(13)  评论(0)  编辑  收藏  举报