Specify a Query

Overview

In this guide, you can learn how to specify a query by using the Kotlin Sync 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 MongoDB uses to match documents in a read or write operation. In a query filter, you can prompt the driver to search for documents with an exact match to your query, or you can compose query filters to express more complex matching criteria.

{ "_id": 1, "name": "apples", "quantity": 5, "rating": 3, "color": "red", "type": ["fuji", "honeycrisp"] },
{ "_id": 2, "name": "bananas", "quantity": 7, "rating": 4, "color": "yellow", "type": ["cavendish"] },
{ "_id": 3, "name": "oranges", "quantity": 6, "rating": 2, "type": ["naval", "mandarin"] },
{ "_id": 4, "name": "pineapple", "quantity": 3, "rating": 5, "color": "yellow" },

data class Fruit(
    @BsonId
    val id: Int,
    val name: String,
    val quantity: Int,
    val rating: Int,
    val color: String,
    val type: List<String>
)

val uri = "<connection string URI>"
val settings = MongoClientSettings.builder()
    .applyConnectionString(ConnectionString(uri))
    .retryWrites(true)
    .build()
val mongoClient = MongoClient.create(settings)
val database = mongoClient.getDatabase("sample_fruit")
val collection = database.getCollection<Fruit>("fruits")
collection.insertMany(listOf(
    Fruit(1, "apples", 5, 3, "red", listOf("fuji", "honeycrisp")),
    Fruit(2, "bananas", 7, 4, "yellow", listOf("cavendish")),
    Fruit(3, "oranges", 6, 2, null, listOf("naval", "mandarin")),
    Fruit(4, "pineapples", 3, 5, "yellow", null)
))

Exact Match

Literal value queries return documents with 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 with a color field value of "yellow".

val results = collection.find(eq(Fruit::color.name, "yellow"))
results.forEach { result ->
    println(result);
}

Fruit(id=2, name=bananas, quantity=7, rating=4, color=yellow, type=[cavendish])
Fruit(id=4, name=pineapples, quantity=3, rating=5, color=yellow, type=null)

Comparison Operators

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

• $gt: Greater than

• $lte: Less than or Equal

• $ne: Not equal

To view a full list of comparison operators, see the Comparison Query Operators guide in the MongoDB Server manual.

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

val results = collection.find(gt(Fruit::rating.name, 2))
results.forEach { result ->
    println(result)
}

Fruit(id=1, name=apples, quantity=5, rating=3, color=red, type=[fuji, honeycrisp])
Fruit(id=2, name=bananas, quantity=7, rating=4, color=yellow, type=[cavendish])
Fruit(id=4, name=pineapples, quantity=3, rating=5, color=yellow, type=null)

Logical Operators

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

• $and, which returns all documents that match the conditions of all clauses

• $or, which returns all documents that match the conditions of one clause

• $nor, which returns all documents that do not match the conditions of any clause

• $not, which returns all documents that do not match the expression

To learn more about logical operators, see the Logical Query Operators guide in the MongoDB Server manual.

The following example specifies a logical operator in a query filter as a parameter to the find() method. The code returns all documents with a quantity field value greater than 5 or a color field value of "yellow".

val results = collection.find(
    or(
        gt(Fruit::quantity.name, 5),
        eq(Fruit::color.name, "yellow")
    )
)
results.forEach { result ->
    println(result)
}

Fruit(id=2, name=bananas, quantity=7, rating=4, color=yellow, type=[cavendish])
Fruit(id=3, name=oranges, quantity=6, rating=2, color=null, type=[naval, mandarin])
Fruit(id=4, name=pineapples, quantity=3, rating=5, color=yellow, type=null)

Array Operators

Array operators match documents based on the value or quantity of elements in an array field. The following is a list of available array operators:

• $all, which returns documents with arrays that contain all elements in the query

• $elemMatch, which returns documents if an element in their array field matches all conditions in the query

• $size, which returns all documents with arrays of a specified size

To learn more about array operators, see the Array Query Operators guide in the MongoDB Server manual.

The following example specifies an array operator in a query filter as a parameter to the find() method. The code returns all documents with a type array field containing exactly 2 elements.

val results = collection.find(size(Fruit::type.name, 2))
results.forEach { result ->
    println(result)
}

Fruit(id=1, name=apples, quantity=5, rating=3, color=red, type=[fuji, honeycrisp])
Fruit(id=3, name=oranges, quantity=6, rating=2, color=null, type=[naval, mandarin])

Element Operators

Element operators query data based on the presence or type of a field.

To learn more about element operators, see the Element Query Operators guide in the MongoDB Server manual.

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.

val results = collection.find(exists(Fruit::color.name))
results.forEach { result ->
    println(result)
}

Fruit(id=1, name=apples, quantity=5, rating=3, color=red, type=[fuji, honeycrisp])
Fruit(id=2, name=bananas, quantity=7, rating=4, color=yellow, type=[cavendish])
Fruit(id=4, name=pineapples, quantity=3, rating=5, color=yellow, type=null)

Evaluation Operators

Evaluation operators return data based on evaluations of either individual fields or the entire collection's documents.

The following is a list of common evaluation operators:

• $text, which performs a text search on the documents

• $regex, which returns documents that match a specified regular expression

• $mod, which performs a modulo operation on the value of a field and returns documents where the remainder is a specified value

To view a full list of evaluation operators, see the Evaluation Query Operators guide in the MongoDB Server manual.

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 with a name field value that has at least two consecutive "p" characters.

val results = collection.find(regex(Fruit::name.name, "p{2,}"))
results.forEach { result ->
    println(result)
}

Fruit(id=1, name=apples, quantity=5, rating=3, color=red, type=[fuji, honeycrisp])
Fruit(id=4, name=pineapples, quantity=3, rating=5, color=yellow, type=null)

Additional Information

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

To learn more about retrieving documents with the Kotlin Sync driver, see Retrieve Data.