query-by-example.adoc

Query by Example

Introduction

This chapter provides an introduction to Query by Example and explains how to use it.

Query by Example (QBE) is a user-friendly querying technique with a simple interface. It allows dynamic query creation and does not require you to write queries that contain field names. In fact, Query by Example does not require you to write queries by using store-specific query languages at all.

Usage

The Query by Example API consists of four parts:

• Probe: The actual example of a domain object with populated fields.

• ExampleMatcher: The ExampleMatcher carries details on how to match particular fields. It can be reused across multiple Examples.

• Example: An Example consists of the probe and the ExampleMatcher. It is used to create the query.

• FetchableFluentQuery: A FetchableFluentQuery offers a fluent API, that allows further customization of a query derived from an Example. Using the fluent API allows you to specify ordering, projection and result processing for your query.

Query by Example is well suited for several use cases:

• Querying your data store with a set of static or dynamic constraints.

• Frequent refactoring of the domain objects without worrying about breaking existing queries.

• Working independently from the underlying data store API.

Query by Example also has several limitations:

• No support for nested or grouped property constraints, such as firstname = ?0 or (firstname = ?1 and lastname = ?2).

• Only supports starts/contains/ends/regex matching for strings and exact matching for other property types.

Before getting started with Query by Example, you need to have a domain object. To get started, create an interface for your repository, as shown in the following example:

Example 1. Sample Person object

public class Person {

  @Id
  private String id;
  private String firstname;
  private String lastname;
  private Address address;

  // … getters and setters omitted
}

The preceding example shows a simple domain object. You can use it to create an Example. By default, fields having null values are ignored, and strings are matched by using the store specific defaults.

Note	Inclusion of properties into a Query by Example criteria is based on nullability. Properties using primitive types (int, double, …) are always included unless ignoring the property path.

Examples can be built by either using the of factory method or by using ExampleMatcher. Example is immutable. The following listing shows a simple Example:

Example 2. Simple Example

Person person = new Person();                         (1)
person.setFirstname("Dave");                          (2)

Example<Person> example = Example.of(person);         (3)

1. Create a new instance of the domain object.

2. Set the properties to query.

3. Create the Example.

You can run the example queries by using repositories. To do so, let your repository interface extend QueryByExampleExecutor<T>. The following listing shows an excerpt from the QueryByExampleExecutor interface:

Example 3. The QueryByExampleExecutor

public interface QueryByExampleExecutor<T> {

  <S extends T> S findOne(Example<S> example);

  <S extends T> Iterable<S> findAll(Example<S> example);

  // … more functionality omitted.
}

Example Matchers

Examples are not limited to default settings. You can specify your own defaults for string matching, null handling, and property-specific settings by using the ExampleMatcher, as shown in the following example:

Example 4. Example matcher with customized matching

Person person = new Person();                          (1)
person.setFirstname("Dave");                           (2)

ExampleMatcher matcher = ExampleMatcher.matching()     (3)
  .withIgnorePaths("lastname")                         (4)
  .withIncludeNullValues()                             (5)
  .withStringMatcher(StringMatcher.ENDING);            (6)

Example<Person> example = Example.of(person, matcher); (7)

1. Create a new instance of the domain object.

2. Set properties.

3. Create an ExampleMatcher to expect all values to match. It is usable at this stage even without further configuration.

4. Construct a new ExampleMatcher to ignore the lastname property path.

5. Construct a new ExampleMatcher to ignore the lastname property path and to include null values.

6. Construct a new ExampleMatcher to ignore the lastname property path, to include null values, and to perform suffix string matching.

7. Create a new Example based on the domain object and the configured ExampleMatcher.

By default, the ExampleMatcher expects all values set on the probe to match. If you want to get results matching any of the predicates defined implicitly, use ExampleMatcher.matchingAny().

You can specify behavior for individual properties (such as "firstname" and "lastname" or, for nested properties, "address.city"). You can tune it with matching options and case sensitivity, as shown in the following example:

Example 5. Configuring matcher options

ExampleMatcher matcher = ExampleMatcher.matching()
  .withMatcher("firstname", endsWith())
  .withMatcher("lastname", startsWith().ignoreCase());
}

Another way to configure matcher options is to use lambdas (introduced in Java 8). This approach creates a callback that asks the implementor to modify the matcher. You need not return the matcher, because configuration options are held within the matcher instance. The following example shows a matcher that uses lambdas:

Example 6. Configuring matcher options with lambdas

ExampleMatcher matcher = ExampleMatcher.matching()
  .withMatcher("firstname", match -> match.endsWith())
  .withMatcher("firstname", match -> match.startsWith());
}

Queries created by Example use a merged view of the configuration. Default matching settings can be set at the ExampleMatcher level, while individual settings can be applied to particular property paths. Settings that are set on ExampleMatcher are inherited by property path settings unless they are defined explicitly. Settings on a property patch have higher precedence than default settings. The following table describes the scope of the various ExampleMatcher settings:

Table 1. Scope of ExampleMatcher settings

Setting	Scope
Null-handling	ExampleMatcher
String matching	ExampleMatcher and property path
Ignoring properties	Property path
Case sensitivity	ExampleMatcher and property path
Value transformation	Property path

Fluent API

QueryByExampleExecutor offers one more method, which we did not mention so far: <S extends T, R> R findBy(Example<S> example, Function<FluentQuery.FetchableFluentQuery<S>, R> queryFunction). As with other methods it executes a query derived from an Example. But with the second argument you can control aspects of that execution, that you can’t control dynamically otherwise. You do so by invoking the various methods of the FetchableFluentQuery in the second argument. sortBy allows you to specify an ordering for your result. as allows you to specify the type to which you want the result to be transformed. project limits the attributes queried. first, firstValue, one, oneValue, all, page, stream, count, exists define what kind of result you’ll get and also how the query behaves when more than the expected number of results are available.

Example 7. Use the fluent API to get the last of potentially many results, ordered by lastname.

Optional<Person> match = repository.findBy(example,
    q -> q
        .sortBy(Sort.by("lastname").descending())
        .first()
);