With ObjectBox DB you use the QueryBuilder class to specify criteria and create Query objects. The Query class will actually run the query and return matching objects.

QueryBuilder

The QueryBuilder<T> class lets you build custom queries for your entities. Create an instance via Box.query() .

QueryBuilder offers several methods to define query conditions for properties of an entity. To specify a property ObjectBox does not use their names as text but meta-information "underscore" classes (like User_) that are generated during build time. The meta-information classes have a static field for each property (like User_.firstName). This allows referencing properties safely with compile-time checks to prevent runtime errors, for example, because of typos.

Here are two examples using a simple and a more complicated query:

Simple condition example: Query for all users with the first name “Joe”:

Query<User> query = userBox.query().equal(User_.firstName, "Joe").build();
List<User> joes = query.find();
query.close();

val query = userBox.query().equal(User_.firstName, "Joe").build()
val joes = query.find()
query.close()
​
// Or shorten with use:
val joes = userBox.query().equal(User_.firstName, "Joe").build()
        .use { it.find() }

Query<User> query = userBox.query(User_.firstName.equals('Joe')).build();
List<User> joes = query.find();
query.close();

Multiple conditions example: Get users with the first name “Joe” that are born later than 1970 and whose last name starts with “O”.

Query<User> query = userBox.query()
        .equal(User_.firstName, "Joe")
        .greater(User_.yearOfBirth, 1970)
        .startsWith(User_.lastName, "O")
        .build();
List<User> youngJoes = query.find();
query.close(); // Or keep the query instance to re-use it

val query = userBox.query()
        .equal(User_.firstName, "Joe")
        .greater(User_.yearOfBirth, 1970)
        .startsWith(User_.lastName, "O")
        .build()
val youngJoes = query.find()
query.close() // Or keep the query instance to re-use it

Query<User> query = userBox.query(
    User_.firstName.equals('Joe') &
    User_.yearOfBirth.greaterThan(1970) &
    User_.lastName.startsWith('O')).build();
List<User> youngJoes = query.find();

New Query API

ObjectBox Java/Kotlin 3.0.0-alpha1+ introduces a new query API that accepts complex nested conditions, e.g. the equivalent of (A or B) and (C or D).

To build a query with the new API use Box.query(condition) and supply a condition built using entity Property methods, like property.equal(value). All properties of an entity can be accessed using its underscore class. For example, for an entity User a property could be User_.firstName, a condition using it could be User_.firstName.equal("Joe").

Simple condition example: Query for all users with the first name “Joe”.

Query<User> query = userBox.query(User_.firstName.equal("Joe")).build();
List<User> joes = query.find();
query.close();

val query = userBox.query(User_.firstName.equal("Joe")).build()
val joes = query.find()
query.close()

Query<User> query = userBox.query(User_.firstName.equals('Joe')).build();
List<User> joes = query.find();
query.close();

Multiple conditions example: Get users with the first name “Joe” that are born later than 1970 and whose last name starts with “O”.

Query<User> query = userBox.query(
        User_.firstName.equal("Joe")
                .and(User_.yearOfBirth.greater(1970))
                .and(User_.lastName.startsWith("O")))
        .build();
List<User> youngJoes = query.find();
query.close();

val query = userBox.query(
        User_.firstName equal "Joe"
                and (User_.yearOfBirth greater 1970)
                and (User_.lastName startsWith "O")
        .build()
val youngJoes = query.find()
query.close()

Query<User> query = userBox.query(
            User_.firstName.equal('Joe')
            .and(User_.yearOfBirth.greaterThan(1970))
            .and(User_.lastName.startsWith('O')))
        .build();
        
// or use operator overloads:
Query<User> query = userBox.query(
            User_.firstName.equal('Joe') &
            User_.yearOfBirth.greaterThan(1970) &
            User_.lastName.startsWith('O'))
        .build();

Conditions are combined with QueryCondition.and(condition) or QueryCondition.or(condition). This implicitly adds parentheses around the combined conditions, e.g. cond1.and(cond2) is logically equivalent to (cond1 AND cond2).

To nest conditions pass a single or combined condition.

// equal AND (less OR oneOf)
Query<User> query = box.query(
        User_.firstName.equal("Joe")
                .and(User_.age.less(12)
                        .or(User_.stamp.oneOf(new long[]{1012}))))
        .order(User_.age)
        .build();

// equal AND (less OR oneOf)
val query = box.query(
        User_.firstName equal "Joe"
                and (User_.age less 12
                or (User_.stamp oneOf longArrayOf(1012))))
        .order(User_.age)
        .build()

Query<User> query = box.query(
    User_.firstName.equal('Joe')
        .and(User_.age.lessThan(12)
        .or(User_.stamp.oneOf([1012]))))
    .order(User_.age)
    .build();

Other notable changes

• property.oneOf(array) replaces the in(property, array) (inValues(property, array) for Kotlin) condition.

• Use Kotlin infix functions to write condition and condition and condition or condition.

• Use Dart operator overloads to write condition & condition and condition | conditon

• Use condition.alias(aliasName) to set an alias for a condition that can later be used with the Query setParameter methods.

Notable conditions

In addition to expected conditions like equal(), notEqual(), greater() and less() there are also conditions like:

• isNull() and notNull(),

• between() to filter for values that are between the given two,

• in() and notIn() to filter for values that match any in the given array,

• startsWith(), endsWith() and contains() for extended String filtering.

In addition, there is and() and or() to build more complex combinations of conditions.

For an overview of all available criteria, please refer to the QueryBuilder class and its method documentations.

Ordering results

In addition to specifying conditions, you can order the returned results using the order() method:

Query<User> query = userBox.query().equal(User_.firstName, "Joe")
    .order(User_.lastName) // in ascending order, ignoring case
    .build();

val query = userBox.query().equal(User_.firstName, "Joe")
    .order(User_.lastName) // in ascending order, ignoring case
    .build()

// in ascending order, ignoring case
final qBuilder = box.query(User_.firstName.equals('Joe')..order(User_.lastName);
final query = qBuilder.build();

You can also pass flags to order() to sort in descending order, to sort case sensitive or to specially treat null values. For example to sort the above results in descending order and case sensitive instead:

.order(User_.lastName, QueryBuilder.DESCENDING | QueryBuilder.CASE_SENSITIVE)

.order(User_.lastName, QueryBuilder.DESCENDING or QueryBuilder.CASE_SENSITIVE)

.order(User_.lastName, flags: Order.descending | Order.caseSensitive)

Order directives can also be chained. Check the method documentation (Java) for details.

Debug queries

To see what query is actually executed by ObjectBox:

// Set the LOG_QUERY_PARAMETERS debug flag
BoxStore store = MyObjectBox.builder()
    .debugFlags(DebugFlags.LOG_QUERY_PARAMETERS)
    .build();
    
// Execute a query
query.find();

// Set the LOG_QUERY_PARAMETERS debug flag
val store = MyObjectBox.builder()
    .debugFlags(DebugFlags.LOG_QUERY_PARAMETERS)
    .build()
    
// Execute a query
query.find()

print(query.describeParameters());

Then in your console (or logcat on Android) you will see log output like:

Parameters for query #2:
(firstName ==(i) "Joe"
 AND age < 12)

Query

Queries are created (and not yet executed) by calling build() on the QueryBuilder.

Query<User> query = builder.build();

Finding objects

There are a couple of find methods to retrieve objects matching the query:

// return all entities matching the query
List<User> joes = query.find();
​
// return only the first result or null if none
User joe = query.findFirst();
​
// return the only result or null if none, throw if more than one result
User joe = query.findUnique();

To return all entities matching the query simply call find().

To only return the first result, use findFirst().

If you expect a unique result, call findUnique() instead. It will give you a single result or null, if no matching entity was found and throw an exception if there was more than one result.

Reusing Queries and Parameters

If you frequently run the same query you should cache the Query object and re-use it. To make a Query more reusable you can change the values, or query parameters, of each condition you added even after the Query is built. Let's see how.

Assume we want to find a list of User with specific firstName values. First, we build a regular Query with an equal() condition for firstName. Because we have to pass an initial parameter value to equal() but plan to override it before running the Query later, we just pass an empty string:

// build query
Query<User> query = userBox.query().equal(User_.firstName, "").build();

Now at some later point we want to actually run the Query. To set a value for the firstName parameter we call setParameter() on the Query and pass the firstName property and the new parameter value:

// change firstName parameter to "Joe", get results
List<User> joes = query.setParameter(User_.firstName, "Joe").find();
...
// change firstName parameter to "Jake", get results
List<User> jakes = query.setParameter(User_.firstName, "Jake").find();

So you might already be wondering, what happens if you have more than one condition using firstName? For this purpose you can assign each parameter an alias by calling parameterAlias() right after specifying the condition:

// assign alias "name" to the equal() query parameter
Query<User> query = userBox.query()
        .equal(User_.firstName, "").parameterAlias("name");

Then, when setting a new parameter value pass the alias instead of the property:

// change parameter with alias "name" to "Joe", get results
List<User> joes = query.setParameter("name", "Joe").find();

Limit, Offset, and Pagination

Sometimes you only need a subset of a query, for example, the first 10 elements to display in your user interface. This is especially helpful (and resource-efficient) when you have a high number of entities and you cannot limit the result using query conditions only.

// offset by 10, limit to at most 5 results
List<User> joes = query.find(10, 5);

// offset by 10, limit to at most 5 results
val joes = query.find(10, 5)

// offset by 10, limit to at most 5 results
query
  ..offset = 10
  ..limit = 5;
List<User> joes = query.find();

offset: The first offset results are skipped.

limit: At most limit results are returned.

Lazy loading results

To avoid loading query results right away, Query offers findLazy() and findLazyCached() which return a LazyList of the query results.

​LazyList is a thread-safe, unmodifiable list that reads entities lazily only once they are accessed. Depending on the find method called, the lazy list will be cached or not. Cached lazy lists store the previously accessed objects to avoid loading entities more than once. Some features of the list are limited to cached lists (e.g. features that require the entire list). See the LazyList class documentation for more details.

Query results stream

Instead of reading the whole result (list of objects) using find() you can stream it using stream() :

Query<User> query = userBox.query().build();
Stream<User stream = query.stream();
await stream.forEach((User user) => print(user));
query.close();

Removing Objects

To remove all objects matching a query, call query.remove() .

PropertyQuery

If you only want to return the values of a particular property and not a list of full objects you can use a PropertyQuery. After building a query, simply call property(Property) to define the property followed by the appropriate find method.

For example, instead of getting all Users, to just get their email addresses:

String[] emails = userBox.query().build()
    .property(User_.email)
    .findStrings();
    
// or use .findString() to return just the first result

val emails = userBox.query().build()
    .property(User_.email)
    .findStrings()
    
// or use .findString() to return just the first result

final query = userBox.query().build();
List<String> emails = query.property(User_.email).find();
query.close();

Handling null values

By default, null values are not returned. However, you can specify a replacement value to return if a property is null:

// includes 'unknown' for each null email
String[] emails = userBox.query().build()
    .property(User_.email)
    .nullValue("unknown")
    .findStrings();

// includes 'unknown' for each null email
val emails = userBox.query().build()
    .property(User_.email)
    .nullValue("unknown")
    .findStrings()

final query = userBox.query().build();
// includes 'unknown' for each null email
List<String> emails = query.property(User_.email).find(replaceNullWith: 'unknown');
query.close();

Distinct and unique results

The property query can also only return distinct values:

PropertyQuery pq = userBox.query().build().property(User_.firstName);
​
// returns ['joe'] because by default, the case of strings is ignored.
String[] names = pq.distinct().findStrings();
​
// returns ['Joe', 'joe', 'JOE']
String[] names = pq.distinct(StringOrder.CASE_SENSITIVE).findStrings();
​
// the query can be configured to throw there is more than one value
String[] names = pq.unique().findStrings();

val pq = userBox.query().build().property(User_.firstName)
​
// returns ['joe'] because by default, the case of strings is ignored.
val names = pq.distinct().findStrings()
​
// returns ['Joe', 'joe', 'JOE']
val names = pq.distinct(StringOrder.CASE_SENSITIVE).findStrings()
​
// the query can be configured to throw there is more than one value
val names = pq.unique().findStrings()

final query = userBox.query().build();
PropertyQuery<String> pq = query.property(User_.firstName);
pq.distinct = true;
​
// returns ['Joe', 'joe', 'JOE'] 
List<String> names = pq.find();
​
// returns ['joe']
pq.caseSensitive = false;
List<String> names = pq.find(); 
query.close();

Aggregating values

Property queries (JavaDoc and Dart API docs) also offer aggregate functions to directly calculate the minimum, maximum, average, sum and count of all found values:

• min() / minDouble(): Finds the minimum value for the given property over all objects matching the query.

• max() / maxDouble(): Finds the maximum value.

• sum() / sumDouble(): Calculates the sum of all values. Note: the non-double version detects overflows and throws an exception in that case.

• avg() : Calculates the average (always a double) of all values.

• count(): returns the number of results. This is faster than finding and getting the length of the result array. Can be combined with distinct() to count only the number of distinct values.

Add query conditions for related entities (links)

After creating a relation between entities, you might want to add a query condition for a property that only exists in the related entity. In SQL this is solved using JOINs. But as ObjectBox is not a SQL database we built something very similar: links. Links are based on Relations - see the doc page for the introduction.

Assume there is a Person that can be associated with multiple Address entities:

@Entity
public class Person {
    @Id long id;
    String name;
    ToMany<Address> addresses;
}
@Entity
public class Address {
    @Id long id;
    String street;
    String zip;
}

@Entity
class Person {
    @Id var id: Long = 0
    var name: String? = null
    lateinit var addresses: ToMany<Address>
}
@Entity
class Address {
    @Id var id: Long = 0
    var street: String? = null
    var zip: String? = null
{

@Entity()
class Person {
    int id;
    String name;
    final addresses = ToMany<Address>();
}
​
@Entity()
class Address {
    int id;
    String street;
    String zip;
}

To get a Person with a certain name that also lives on a specific street, we need to query the associated Address entities of a Person. To do this, use the link() method of the query builder to tell that the addresses relation should be queried. Then add a condition for Address:

// get all Person objects named "Elmo"...
QueryBuilder<Person> builder = personBox
        .query().equal(Person_.name, "Elmo");
// ...which have an address on "Sesame Street"
builder.link(Person_.addresses).equal(Address_.street, "Sesame Street");
List<Person> elmosOnSesameStreet = builder.build().find();

// get all Person objects named "Elmo"...
val builder = personBox.query().equal(Person_.name, "Elmo")
// ...which have an address on "Sesame Street"
builder.link(Person_.addresses).equal(Address_.street, "Sesame Street")
val elmosOnSesameStreet = builder.build().find()

// get all Person objects named "Elmo"...
QueryBuilder<Person> builder = personBox.query(Person_.name.equals('Elmo'));
// ...which have an address on "Sesame Street"
builder.linkMany(Person_.addresses, Address_.street.equals('Sesame Street'));
Query<Person> query = builder.build();
List<Person> elmosOnSesameStreet = query.find();
query.close();

What if we want to get a list of Address instead of Person? If you know ObjectBox relations well, you would probably add a @Backlink relation to Address and build your query using it with link() as shown above:

@Entity
public class Address {
    // ...
    @Backlink(to = "addresses")
    ToMany<Person> persons;
}
​
// get all Address objects with street "Sesame Street"...
QueryBuilder<Address> builder = addressBox
        .query().equal(Address_.street, "Sesame Street");
// ...which are linked from a Person named "Elmo"
builder.link(Address_.persons).equal(Person_.name, "Elmo");
List<Address> sesameStreetsWithElmo = builder.build().find();

@Entity
class Address {
    // ...
    @Backlink(to = "addresses")
    lateinit var persons: ToMany<Person>
}
​
// get all Address objects with street "Sesame Street"...
val builder = addressBox.query().equal(Address_.street, "Sesame Street")
// ...which are linked from a Person named "Elmo"
builder.link(Address_.persons).equal(Person_.name, "Elmo")
val sesameStreetsWithElmo = builder.build().find()

@Entity()
class Address {
    ...
    
    @Backlink()
    final persons = ToMany<Person>();
}
​
// get all Address objects with street "Sesame Street"...
QueryBuilder<Address> builder = 
    addressBox.query(Address_.street.equals('Sesame Street'));
// ...which are linked from a Person named "Elmo"
builder.linkMany(Address_.persons, Person_.name.equals('Elmo'));
Query<Address> query = builder.build();
List<Address> sesameStreetsWithElmo = query.find();
query.close();

But actually, you do not have to modify the Address entity (you still can if you need the @Backlink elsewhere). Instead, we can use the backlink() method to create a backlink to the addresses relation from Person just for that query:

// get all Address objects with street "Sesame Street"...
QueryBuilder<Address> builder = addressBox.query()
    .equal(Address_.street, "Sesame Street");
// ...which are linked from a Person named "Elmo"
builder.backlink(Person_.addresses).equal(Person_.name, "Elmo");
List<Address> sesameStreetsWithElmo = builder.build().find();

// get all Address objects with street "Sesame Street"...
val builder = addressBox.query().equal(Address_.street, "Sesame Street")
// ...which are linked from a Person named "Elmo"
builder.backlink(Person_.addresses).equal(Person_.name, "Elmo")
val sesameStreetsWithElmo = builder.build().find()

// get all Address objects with street "Sesame Street"...
QueryBuilder<Address> builder = 
    addressBox.query(Address_.street.equals('Sesame Street'));
// ...which are linked from a Person named "Elmo"
builder.backlinkMany(Person_.addresses, Person_.name.equals('Elmo'));
Query<Address> query = builder.build();
List<Address> sesameStreetsWithElmo = query.find();
query.close();

Eager Loading of Relations

By default relations are loaded lazily: when you first access a ToOne or ToMany property it will perform a database lookup to get its data. On each subsequent access it will use a cached version of that data.