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Projections

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 in the query operation.

Projection Document

The projection document limits the fields to return for all matching documents. The projection document can specify the inclusion of fields or the exclusion of field and has the following form:

{ 'projection': { field1: <value>, field2: <value> ... } }

<value> may be 0 (or false) to exclude the field, or 1 (or true) to include it. With the exception of the _id field, you may not have both inclusions and exclusions in the same projection document.

Examples

The following code example uses the restaurants sample dataset.

To return only the name, cuisine and _id fields for documents that match the query filter, explicitly include the name and cuisine fields in the projection document. The _id field is included automatically unless specifically excluded.

client = Mongo::Client.new([ '127.0.0.1:27017' ], :database => 'test')
collection = client[:restaurants]

collection.find({}, { 'projection' =>
  { 'name' => 1, 'cuisine' => 1 } }).limit(5).each do |doc|
  p doc
end

To return name and cuisine but exclude all other fields, including _id, use the following projection document:

{ 'projection' => { 'name' => 1, 'cuisine' => 1, '_id' => 0 } }

To return all fields except the address field, use the following:

{ 'projection' => { 'address' => 0 } }
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