Add MongoDB example for linking documents.

Closes #631

Author: christophstrobl

README.md

@@ -72,6 +72,7 @@ Local Elasticsearch instance must be running to run the tests.
 * `gridfs` - Example project showing usage of gridFS with MongoDB.
 * `jmolecules` - Example of Spring Data MongoDB working with a jMolecules based domain model.
 * `kotlin` - Example for using [Kotlin](https://kotlinlang.org/) with MongoDB.
+* `linking` - Example demonstrating possibilities for linking documents.
 * `query-by-example` - Example project showing usage of Query by Example with MongoDB.
 * `querydsl` - Example project showing imperative and reactive [Querydsl](https://github.com/querydsl/querydsl) support for MongoDB.
 * `reactive` - Example project to show reactive template and repository support.

mongodb/linking/README.md

@@ -0,0 +1,62 @@
+# Spring Data MongoDB - Linking Example
+
+This project contains examples for linking Documents using:
+
+* `@DBRef`
+* `@DocumentReference`
+
+## DBRef
+
+Use MongoDB native `dbref` for storing associations.
+
+```java
+class Manager {
+	
+    @DBRef
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+## DocumentReference
+
+Link documents via their id.
+
+```java
+class Manager {
+	
+    @DocumentReference
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+Link documents via a (non _id_) property.
+
+```java
+class Manager {
+	
+    @DocumentReference(lookup = "{ 'name' : ?#{#target} }")
+    private List<Employee> employees;
+
+    // ...
+}
+```
+
+Link documents JPA style.
+
+```java
+class Manager {
+
+    @ReadOnlyProperty
+    @DocumentReference(lookup="{'managerId':?#{#self._id} }")
+    private List<Employee> employees;
+
+    // ...
+}
+```
+----
+
+For more information please refer to the [Reference Documentation](https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#mapping-usage-references).

mongodb/linking/pom.xml

@@ -0,0 +1,15 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+
+	<artifactId>spring-data-mongodb-linking-example</artifactId>
+
+	<name>Spring Data MongoDB - Linking Example</name>
+
+	<parent>
+		<groupId>org.springframework.data.examples</groupId>
+		<artifactId>spring-data-mongodb-examples</artifactId>
+		<version>2.0.0.BUILD-SNAPSHOT</version>
+	</parent>
+
+</project>

mongodb/linking/src/main/java/example/springdata/mongodb/linking/dbref/ApplicationConfiguration.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.linking.dbref;
+
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/**
+ * @author Christoph Strobl
+ */
+@SpringBootApplication
+class ApplicationConfiguration {}

mongodb/linking/src/main/java/example/springdata/mongodb/linking/dbref/Employee.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.linking.dbref;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.mapping.DBRef;
+
+/**
+ * @author Christoph Strobl
+ */
+class Employee {
+
+	@Id
+	private String id;
+	private String name;
+
+	@DBRef(lazy = true) // lazy to avoid stackoverflow on load
+	private Manager manager;
+
+	public Employee() {
+	}
+
+	public Employee(String id, String name) {
+		this.id = id;
+		this.name = name;
+	}
+
+	public String getId() {
+		return id;
+	}
+
+	public void setId(String id) {
+		this.id = id;
+	}
+
+	public String getName() {
+		return name;
+	}
+
+	public void setName(String name) {
+		this.name = name;
+	}
+
+	public Manager getManager() {
+		return manager;
+	}
+
+	public void setManager(Manager manager) {
+		this.manager = manager;
+	}
+}

mongodb/linking/src/main/java/example/springdata/mongodb/linking/dbref/Manager.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.linking.dbref;
+
+import java.util.List;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.mapping.DBRef;
+
+/**
+ * @author Christoph Strobl
+ */
+class Manager {
+
+	@Id //
+	private String id;
+	private String name;
+
+	@DBRef //
+	private List<Employee> employees;
+
+	public Manager() {
+	}
+
+	public Manager(String id, String name) {
+		this.id = id;
+		this.name = name;
+	}
+
+	public String getId() {
+		return id;
+	}
+
+	public void setId(String id) {
+		this.id = id;
+	}
+
+	public String getName() {
+		return name;
+	}
+
+	public void setName(String name) {
+		this.name = name;
+	}
+
+	public List<Employee> getEmployees() {
+		return employees;
+	}
+
+	public void setEmployees(List<Employee> employees) {
+		this.employees = employees;
+	}
+}

mongodb/linking/src/main/java/example/springdata/mongodb/linking/docref/jpastyle/ApplicationConfiguration.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.linking.docref.jpastyle;
+
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/**
+ * @author Christoph Strobl
+ */
+@SpringBootApplication
+class ApplicationConfiguration {
+
+}

mongodb/linking/src/main/java/example/springdata/mongodb/linking/docref/jpastyle/Employee.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.springdata.mongodb.linking.docref.jpastyle;
+
+import org.springframework.data.annotation.Id;
+
+/**
+ * @author Christoph Strobl
+ */
+class Employee {
+
+	@Id
+	private String id;
+	private String name;
+
+	/**
+	 * Only hold the id of the manager to link to it.
+	 */
+	private String managerId;
+
+	public Employee() {
+	}
+
+	public Employee(String id, String name) {
+		this.id = id;
+		this.name = name;
+	}
+
+	public String getId() {
+		return id;
+	}
+
+	public void setId(String id) {
+		this.id = id;
+	}
+
+	public String getName() {
+		return name;
+	}
+
+	public void setName(String name) {
+		this.name = name;
+	}
+
+	public String getManagerId() {
+		return managerId;
+	}
+
+	public void setManagerId(String managerId) {
+		this.managerId = managerId;
+	}
+}