如何限制 Neo4j 中两个节点之间只有一种关系? [英] How can I limit to only one relationship between two nodes in Neo4j?

问题描述

我有以下图表:

目前我正在使用这个 QUERY 添加两个节点之间的关系:

Currently I am using this QUERY to add a relationship between two nodes:

MATCH (a:Service),(b:Service)
WHERE a.service_id = 'cs2322' and b.service_id = 'ab3232'
CREATE (a)-[r:DEPENDENT_ON]->(b)
RETURN type(r)

但是我不想在任何两个节点之间有多个关系，因为我想可视化我的服务以及它们之间的依赖关系，所以我不能让一个服务两次依赖另一个.

However I dont want to have more than one relationship between any two nodes, because I want to visualise my services and the dependency between them, so I cannot have a service being two times dependent on the other.

如果我尝试在两个节点之间创建关系，而这两个节点之间已经在每个方向上建立了关系，那么有什么方法可以限制它以强制 Neo4j 服务器抛出错误?

Is there any way I can limit this to force neo4j server to throw an error if I try to create a relationship between two nodes which already have a relationship per direction with one-another?

推荐答案

如果创建重复的关系，没有内置的方法可以抛出错误.但这也是执行此类政策的一种非常昂贵的方式.

There is no built-in way to throw an error if you create a duplicate relationship. But that would also be a pretty expensive way to enforce such a policy.

相反，您可以使用 MERGE 而不是 CREATE 以避免创建重复的关系.

Instead, you can use MERGE instead of CREATE to avoid creating duplicate relationships.

例如，如果DEPENDENT_ON 关系不存在，这个查询只会创建它；否则，它只会将现有关系绑定到 r:

For example, this query will only create the DEPENDENT_ON relationship if it does not already exist; otherwise, it will just bind the existing relationship to r:

    MATCH (a:Service), (b:Service)
    WHERE a.service_id = 'cs2322' AND b.service_id = 'ab3232'
    MERGE (a)-[r:DEPENDENT_ON]->(b)
    RETURN TYPE(r)

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