Specify a Query

Overview

In this guide, you can learn how to specify a query by using the Ruby driver.

You can refine the set of documents that a query returns by creating a query filter. A query filter is an expression that specifies the search criteria that MongoDB uses to match documents in a read or write operation. In a query filter, you can prompt the driver to search for documents that have an exact match to your query, or you can compose query filters to express more complex matching criteria.

database = client.use('db')
collection = database[:fruits]
# Inserts documents representing fruits
fruits = [
{ _id: 1, name: 'apples', qty: 5, rating: 3, color: 'red', type: ['fuji', 'honeycrisp'] },
{ _id: 2, name: 'bananas', qty: 7, rating: 4, color: 'yellow', type: ['cavendish'] },
{ _id: 3, name: 'oranges', qty: 6, rating: 2, type: ['naval', 'mandarin'] },
{ _id: 4, name: 'pineapples', qty: 3, rating: 5, color: 'yellow' }
]
collection.insert_many(fruits)

Exact Match

Literal value queries return documents that have an exact match to your query filter.

The following example specifies a query filter as a parameter to the find method. The code returns all documents in which the value of the color field is 'yellow':

filter = { color: 'yellow' }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>2, "name"=>"bananas", "qty"=>7, "rating"=>4, "color"=>"yellow", "type"=>["cavendish"]}
{"_id"=>4, "name"=>"pineapples", "qty"=>3, "rating"=>5, "color"=>"yellow"}

results = collection.find

Comparison Operators

Comparison operators evaluate a document field value against a specified value in your query filter. The following list describes common comparison operators:

• $gt: Returns documents in which the value of the given field is greater than the specified value

• $lte: Returns documents in which the value of the given field is less than or equal to the specified value

• $ne: Returns documents in which the value of the given field does not equal the specified value

The following example specifies a comparison operator in a query filter as a parameter to the find method. The code returns all documents that have a rating field value greater than 2:

filter = { rating: { '$gt' => 2 } }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>1, "name"=>"apples", "qty"=>5, "rating"=>3, "color"=>"red", "type"=>["fuji", "honeycrisp"]}
{"_id"=>2, "name"=>"bananas", "qty"=>7, "rating"=>4, "color"=>"yellow", "type"=>["cavendish"]}
{"_id"=>4, "name"=>"pineapples", "qty"=>3, "rating"=>5, "color"=>"yellow"}

Logical Operators

Logical operators match documents by using logic applied to the results of two or more sets of expressions. The following list describes each logical operator:

• $and: Returns documents that match the conditions of all clauses

• $or: Returns documents that match the conditions of one clause

• $nor: Returns documents that do not match the conditions of any clause

• $not: Returns documents that do not match the expression

The following example specifies a logical operator in a query filter as a parameter to the find method. The code returns all documents that have a qty field value greater than 5 or a color field value of 'yellow':

filter = { '$or' => [{ qty: { '$gt' => 5 } }, { color: 'yellow' }] }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>2, "name"=>"bananas", "qty"=>7, "rating"=>4, "color"=>"yellow", "type"=>["cavendish"]}
{"_id"=>3, "name"=>"oranges", "qty"=>6, "rating"=>2, "type"=>["naval", "mandarin"]}
{"_id"=>4, "name"=>"pineapples", "qty"=>3, "rating"=>5, "color"=>"yellow"}

Array Operators

Array operators match documents based on the value or quantity of elements in an array field. The following list describes each array operator:

• $all: Returns documents with arrays that contain all elements in the query

• $elemMatch: Returns documents if an element in their array field matches all conditions in the query

• $size: Returns documents with arrays of a specified size

The following example specifies an array operator in a query filter as a parameter to the find method. The code returns all documents in which the type array field contains 2 elements:

filter = { type: { '$size' => 2 } }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>1, "name"=>"apples", "qty"=>5, "rating"=>3, "color"=>"red", "type"=>["fuji", "honeycrisp"]}
{"_id"=>3, "name"=>"oranges", "qty"=>6, "rating"=>2, "type"=>["naval", "mandarin"]}

Element Operators

Element operators query data based on the presence or type of a field. The following list describes each element operator:

• $exists: Returns documents that contain the specified field

• $type: Returns documents that contain a field of the specified type

The following example specifies an element operator in a query filter as a parameter to the find method. The code returns all documents that have a color field:

filter = { color: { '$exists' => true } }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>1, "name"=>"apples", "qty"=>5, "rating"=>3, "color"=>"red", "type"=>["fuji", "honeycrisp"]}
{"_id"=>2, "name"=>"bananas", "qty"=>7, "rating"=>4, "color"=>"yellow", "type"=>["cavendish"]}
{"_id"=>4, "name"=>"pineapples", "qty"=>3, "rating"=>5, "color"=>"yellow"}

Evaluation Operators

Evaluation operators return data based on evaluations of either individual fields or the entire collection's documents. The following list describes common element operators:

• $text: Performs a text search on the documents

• $regex: Returns documents that match a specified regular expression

• $mod: Performs a modulo operation on the value of a field and returns documents where the remainder is a specified value

The following example specifies an evaluation operator in a query filter as a parameter to the find method. The code uses a regular expression to return all documents in which the name field value has at least two consecutive 'p' characters:

filter = { name: /p{2,}/ }
results = collection.find(filter)
results.each do |doc|
  puts doc
end

{"_id"=>1, "name"=>"apples", "qty"=>5, "rating"=>3, "color"=>"red", "type"=>["fuji", "honeycrisp"]}
{"_id"=>4, "name"=>"pineapples", "qty"=>3, "rating"=>5, "color"=>"yellow"}

Additional Information

To learn more about querying documents, see Query Documents in the MongoDB Server manual.

To learn more about retrieving documents by using the Ruby driver, see the Retrieve Data guide.