Neo4j OGM 2.0开发指南：从入门到深度应用

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Neo4j OGM Manual 2.0 是一个针对 Neo4j 2.0 版本的官方对象图映射库指南，由 Neo4j 团队编撰。该手册分为三个部分：介绍、教程和参考文档，旨在帮助开发者理解和利用 Neo4j 的图形数据库特性以及如何将其与对象关系模型（Object-Graph Mapping, OGM）相结合。 1. **介绍**：这部分首先定义了什么是图形数据库，强调其在存储和检索复杂网络数据方面的优势。它指出，图形数据库通过节点（Nodes）和关系（Relationships）高效地组织数据，并支持高性能查询。 2. **什么是图数据库**：图数据库是一种专门设计用于处理大量相互关联数据的存储引擎，它们能够轻松处理非结构化的、多对多的数据模型，非常适合于社交网络、推荐系统和各种领域中的复杂实体和交互关系。 3. **什么是 OGM**：OGM 是一种工具，它允许开发者将面向对象的编程语言中的类映射到图数据库中的节点和边。它简化了数据模型转换的过程，使得在图形数据库中操作数据就像在传统的关系型数据库中一样直观。 4. **关于 Neo4j**：这部分介绍了 Neo4j，它是目前最流行的开源图数据库之一，以其强大的 Cypher 查询语言和灵活的模式适应性而闻名。Neo4j OGM 作为其官方库，提供了无缝连接 Java 和 Neo4j 的接口。 5. **查询图形数据**：章节详细讲解了如何使用 Cypher 查询语言来探索和操作 Neo4j 数据库，这对于理解并操作图数据至关重要。 6. **索引**：索引在图数据库中扮演着关键角色，这部分可能涉及如何创建、管理和优化索引来提升查询性能。 7. **教程**：教程部分引导读者逐步入门，通过实际示例展示如何构建和管理对象图模型，包括定义领域模型、使用注解和配置 Neo4j OGM。 8. **配置和会话**：这部分可能涵盖了如何配置 Neo4j OGM 以适应不同的应用场景，比如高可用性设置，以及如何管理会话以进行数据操作。 9. **结论**：总结了学习的重要概念和实践技巧，强调了 Neo4j OGM 在实际项目中的价值。 10. **参考文档**：提供完整的 API 文档、技术细节和最佳实践，以便开发者在遇到问题时查阅。整个文档是开发者深入理解 Neo4j 2.0 版本及其 OGM 库的宝贵资源，涵盖了从基础知识到高级用法的全方位指导，无论你是初次接触还是经验丰富的开发者，都能从中获益匪浅。

Neo4j OGM

To simplify development, we’re going to use Neo4j OGM, an object-graph mapping library. Much like

JPA, we’ll be annotating our POJOs and this will map them to nodes, relationships and properties in the

graph.

Neo4j OGM 1.x works against server based installations of Neo4j and uses Cypher over the

transactional HTTP endpoint. Neo4j OGM 2.x supports both server mode (HTTP and bolt) and

embedded Neo4j.

Our sample application will use server mode with HTTP.

While the OGM takes care of boilerplate CRUD operations, it also provides us with the flexibility of

writing our own Cypher queries and controlling persistence depth as we shall see later in this tutorial.

Annotations

Neo4j OGM supports mapping annotated and non-annotated objects models. It’s possible to save any

POJO with the exception of a Relationship Entity without annotations to the graph. The framework will

then apply conventions to decide what to do.

Maven dependency

Before we can use the library, we need to add a dependency.

Maven dependencies for Neo4j OGM 2.x

Ê <groupId>org.neo4j</groupId>

Ê <artifactId>neo4j-ogm-core</artifactId>

Ê <version>{version}</version>

</dependency>

Ê <groupId>org.neo4j</groupId>

Ê <artifactId>neo4j-ogm-http-driver</artifactId>

Ê <version>{version}</version>

</dependency>

Gradle dependencies for Neo4j OGM 2.x

compile "org.neo4j:neo4j-ogm-core:{version}"

compile "org.neo4j:neo4j-ogm-http-driver:{version}"

Refer to Setup for more information on dependencies.

Note that the dependencies for Neo4j OGM 1.x differ -

Maven dependencies for Neo4j OGM 1.x

Ê <groupId>org.neo4j</groupId>

Ê <artifactId>neo4j-ogm</artifactId>

Ê <version>{version}</version>

</dependency>

Gradle dependencies for Neo4j OGM 1.x

compile "org.neo4j:neo4j-ogm:{version}"

Nodes, properties & labels

POJOs annotated with @NodeEntity will be represented as nodes in the graph. The label assigned to

this node can be specified via the label property on the annotation; if not specified, it will default to

the simple class name of the entity. Each parent class in addition also contributes a label to the entity

(with the exception of java.lang.Object). This is useful when we want to retrieve collections of super

types.

Let’s go ahead and annotate all our node entities. Note that we’re overriding the default label for a

Course with Class

@NodeEntity

public class Department {

Ê private String name;

Ê private Set<Subject> subjects;

}

@NodeEntity

public class Subject {

Ê private String name;

Ê private Department department;

Ê private Set<Teacher> teachers;

Ê private Set<Course> courses;

}

@NodeEntity

public class Teacher {

Ê private String name;

Ê private Set<Course> courses;

Ê private Set<Subject> subjects;

}

@NodeEntity(label="Class")

public class Course {

Ê private String name;

Ê private Subject subject;

Ê private Teacher teacher;

Ê private Set<Enrollment> enrollments;

}

@NodeEntity

public class Student {

Ê private String name;

Ê private Set<Enrollment> enrollments;

Ê private Set<StudyBuddy> studyBuddies;

}

Runtime managed labels

As stated above, the label applied to a node is the contents of the @NodeEntity label property, or if not

specified, it will default to the simple class name of the entity. Sometimes it might be necessary to add

and remove additional labels to a node at runtime. We can do this using the @Labels annotation. Let’s

provide a facility for adding additional labels to the Student entity:

@NodeEntity

public class Student {

Ê @Labels

Ê private List<String> labels = new ArrayList<>();

}

Now, upon save, the node’s labels will correspond to the entity’s class hierarchy plus whatever the

contents of the backing field are. We can use one @Labels field per class hierarchy - it should be

exposed or hidden from sub-classes as appropriate.

Relationships

Next up, the relationships between the nodes. Every field in an entity that references another entity is

backed by a relationship in the graph. The @Relationship annotation allows you to specify both the

type of the relationship and the direction. By default, the direction is assumed to be OUTGOING and the

type is the UPPER_SNAKE_CASE field name. We’re going to be specific about the relationship type to

avoid using the default and also make it easier to refactor classes later by not being dependent on the

field name.

@NodeEntity

public class Department {

Ê private String name;

Ê @Relationship(type = "CURRICULUM")

Ê private Set<Subject> subjects;

}

@NodeEntity

public class Subject {

Ê private String name;

Ê @Relationship(type="CURRICULUM", direction = Relationship.INCOMING)

Ê private Department department;

Ê @Relationship(type = "TAUGHT_BY")

Ê private Set<Teacher> teachers;

Ê @Relationship(type = "SUBJECT_TAUGHT", direction = "INCOMING")

Ê private Set<Course> courses;

}

@NodeEntity

public class Teacher {

Ê private String name;

Ê @Relationship(type="TEACHES_CLASS")

Ê private Set<Course> courses;

Ê @Relationship(type="TAUGHT_BY", direction = Relationship.INCOMING)

Ê private Set<Subject> subjects;

}

@NodeEntity(label="Class")

public class Course {

Ê private String name;

Ê @Relationship(type= "SUBJECT_TAUGHT")

Ê private Subject subject;

Ê @Relationship(type= "TEACHES_CLASS", direction=Relationship.INCOMING)

Ê private Teacher teacher;

Ê @Relationship(type= "ENROLLED", direction=Relationship.INCOMING)

Ê private Set<Enrollment> enrollments = new HashSet<>();

}

@NodeEntity

public class Student {

Ê private String name;

Ê @Relationship(type = "ENROLLED")

Ê private Set<Enrollment> enrollments;

Ê @Relationship(type = "BUDDY", direction = Relationship.INCOMING)

Ê private Set<StudyBuddy> studyBuddies;

}

Relationship entities

We have one more entity, and that is the Enrollment. As discussed earlier, this is a relationship entity

since it manages the underlying ENROLLED relation between a student and course. It isn’t a simple

relation because it has a relationship property called enrolledDate.

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