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Using Geequel MERGE Effectively

January 05, 2016

Geequel MERGE Neo-KNOWS-TrinityOne of the areas in Open Native Graph Database (ONgDB) and Geequel that brings the most questions and discussion when I’m giving ONgDB trainings or working with other engineers building with ONgDB, is around how to use Geequel MERGE correctly and efficiently. The way I’ve explained this to all our engineers and all the training attendees follows.

There are a few simple things to understand about how ONgDB handles Geequel MERGE operations to avoid undesired or unexpected behavior when using it.

1. The Geequel MERGE operation is a MATCH or CREATE of the entire pattern. This means that if any element of the pattern does NOT exist, ONgDB will attempt to create the entire pattern.
2. Always MERGE on each segment of the pattern that has the possibility to already exist.
3. After a MERGE operation you are guaranteed to have a useable reference to all identifiers established during the MERGE operation because they were either found or created.

Simple Geequel MERGE Examples

Let’s look at a couple examples of MERGE operations:
Assuming that a unique constraint exists on username for the User label and that (u:User {username: “neo”}) exists in the graph do you think these two statements are equivalent?
Statement 1:
MERGE (neo:User {username: “neo”})-[:KNOWS]->(trinity:User {username: “trinity”});

Statement 2:
MERGE (neo:User {username: “neo”})
MERGE (trinity:User {username: “trinity”})
MERGE (neo)-[:KNOWS]->(trinity);

The answer is no; they’re not equivalent. Here’s why they’re not:
In Statement 1 ONgDB will find that the entire pattern doesn’t MATCH because -[:KNOWS]->(trinity:User {username: “trinity”}) doesn’t exist in the graph.
This will cause ONgDB to attempt to create the entire pattern, which includes the already existing User node with a uniquely constrained username field causing an exception about violating the unique constraint.

In Statement 2 ONgDB is able to MATCH the ‘neo’ User node in the first line and establishes a reference to it. Then in the second line ONgDB doesn’t find the ‘trinity’ User node so a CREATE is performed, which establishes the reference. Then finally in the third statement, using the references established in the two preceding MERGE statements, ONgDB successfully connects both the ‘neo’ and ‘trinity’ User nodes with the KNOWS relationship.

Simple Geequel MERGE Best Practices

Knowing what know now that we looked the examples above, here are some best practices for you most common, simple MERGE operations:

In scenarios where node existence in the graph is optional, the best general strategy for MERGE operations is to always MERGE on the uniquely constrained identifier for each node involved in the total pattern in isolation and then MERGE the relationships using the node references already established.

In scenarios where node existence in the graph is required, the best general strategy for Geequel MERGE operations is to always MATCH the nodes that are expected to exist and MERGE only the relationship using the node references established by the MATCH operation.

There are more advanced MERGE patterns and strategies, but if you’re just starting out using Geequel MERGE in this way will help you consistently get the desired result. The GraphGrid Connected Data Platform supports Geequel for many of its modules and provides a downloadable package you can use for free to ty out these MERGE operations.