Specify a Query

Overview

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

This guide uses the Flux Publisher, which is a Publisher implementation from the Project Reactor library. In Java Reactive Streams, you must use Publisher implementations to control how data is transmitted in your application. To learn more about the Flux class, see Flux in the Project Reactor documentation.

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", "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": "pineapple", "qty": 3, "rating": 5, "color": "yellow" },

import com.mongodb.ConnectionString;
import com.mongodb.MongoClientSettings;
import com.mongodb.ServerApi;
import com.mongodb.ServerApiVersion;
import com.mongodb.client.result.InsertManyResult;
import org.bson.Document;
import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;
import com.mongodb.reactivestreams.client.MongoClient;
import com.mongodb.reactivestreams.client.MongoClients;
import com.mongodb.reactivestreams.client.MongoDatabase;
import com.mongodb.reactivestreams.client.MongoCollection;
import java.util.Arrays;
import java.util.List;
public class QueryDatabase {
    public static void main(String[] args) {
        // Replace the placeholder with your Atlas connection string
        String uri = "<connection string>";
        // Construct a ServerApi instance using the ServerApi.builder() method
        ServerApi serverApi = ServerApi.builder()
                .version(ServerApiVersion.V1)
                .build();
        MongoClientSettings settings = MongoClientSettings.builder()
                .applyConnectionString(new ConnectionString(uri))
                .serverApi(serverApi)
                .build();
        // Create a new client and connect to the server
        try (MongoClient mongoClient = MongoClients.create(settings)) {
            MongoDatabase database = mongoClient.getDatabase("sample_fruits");
            MongoCollection<Document> fruits = database.getCollection("fruits");
            Document document1 = new Document("_id", "1")
                    .append("name", "apples")
                    .append("qty", 5)
                    .append("rating", 3)
                    .append("color", "red")
                    .append("type", Arrays.asList("fuji", "honeycrisp"));
            Document document2 = new Document("_id", "2")
                    .append("name", "bananas")
                    .append("qty", 7)
                    .append("rating", 4)
                    .append("color", "yellow")
                    .append("type", Arrays.asList("cavendish"));
            Document document3 = new Document("_id", "3")
                    .append("name", "oranges")
                    .append("qty", 6)
                    .append("rating", 2)
                    .append("type", Arrays.asList("naval", "mandarin"));
            Document document4 = new Document("_id", "4")
                    .append("name", "pineapple")
                    .append("qty", 3)
                    .append("rating", 5)
                    .append("color", "yellow");
            List<Document> documents = Arrays.asList(document1, document2, document3, document4);
            Publisher<InsertManyResult> insertPublisher = fruits.insertMany(documents);
            Mono.from(insertPublisher).block();                                                        
        }
    }
}

Exact Match

Literal value queries return documents with an exact match to your query filter. To return documents with an exact match, use the eq() comparison operator method.

The following example specifies an eq() comparison operator method as the query filter parameter in the find() method. The code returns all documents with a color field value of "yellow".

FindPublisher<Document> findDocPublisher = fruits.find(eq("color", "yellow"));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_id': 2, 'name': 'bananas', 'qty': 7, 'rating': 4, 'color': 'yellow', 'type': ['cavendish']}
{'_id': 4, 'name': 'pineapple', 'qty': 3, 'rating': 5, 'color': 'yellow'}

FindPublisher<Document> findDocPublisher = fruits.find();
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

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 operator methods:

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

FindPublisher<Document> findDocPublisher = fruits.find(gt("rating", 2));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_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': 'pineapple', '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 is a list of logical operator methods:

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

FindPublisher<Document> findDocPublisher = fruits.find(
        or(gt("qty", 5), eq("color", "yellow")));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_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': 'pineapple', '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 is a list of available array operator methods:

• 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 a size() array operator method in a query filter as a parameter to the find() method. The code returns all documents with a type array field containing 2 elements.

FindPublisher<Document> findDocPublisher = fruits.find(size("type", 2));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_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 is a list of available element operator methods:

• exists(), which returns documents with the specified field

• type(), which returns documents if a field is of the specified type

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

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

FindPublisher<Document> findDocPublisher = fruits.find(exists("color", true));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_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': 'pineapple', '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 is a list of common evaluation operator methods:

• 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 a regex() evaluation operator method 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.

FindPublisher<Document> findDocPublisher = fruits.find(regex("name", "p{2,}"));
Document findResults = Flux.from(findDocPublisher)
         .doOnNext(System.out::println)
         .blockLast();

{'_id': 1, 'name': 'apples', 'qty': 5, 'rating': 3, 'color': 'red', 'type': ['fuji', 'honeycrisp']}
{'_id': 4, 'name': 'pineapple', 'qty': 3, 'rating': 5, 'color': 'yellow'}

Additional Information

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