Why neo4j don't allows not directed or bidirectional relationships at creation time?
cypher, neo4j
Solution
Fundamentally, it boils down to the internals of how the data is stored on disk in Neo4j -- note Chapter 6 of the O'Reilly Neo4j e-book.
In the data structure of a relationship they have a "firstNode" and a "secondNode", where each is either the left or the right hand side of the relationship.
To flag a relationship as uni/bi-directional would require an additional bit per node, where I would argue it is better to retain the direction in the data store and just ignore direction during querying.
Problem
I know that Neo4j requires a relationship direction at creation time, but allows ignore this direction in query time. By this way I can query my graph ignoring the relationship direction. I also know that there are some workarounds for cases when the relationships are naturally bidirectional or not directed, like described here. My question is: Why is it implemented that way? Has a good reason to not allow not directed or bidirectional relationships at creation time? Is it a limitation of the database architecture? The Cypher statements like below are not allowed: ``` CREATE ()-[:KNOWS]-() CREATE ()<-[:KNOWS]->() ``` I searched the web for an answer, but I did not find much. For example, this github issue. Is strange to have to define a relationship direction to one that don't have it. It seems to me that i'm hurting the semantic of my graph. EDIT 1: To clarify my standpoint about a "semantic problem" (maybe the term is wrong): Suppose that I run this simple `CREATE` statement: ``` CREATE (a:Person {name:'a'})-[:KNOWS]->(b:Person {name:'b'}) ``` As result i have this very simple graph: The `:KNOWS` relationship has a direction only because Neo4j requires a relationship direction at creation time. In my domain `a` knows `b` and `b` knows `a`. Now, a new team member will query my graph with this Cypher query: ``` MATCH path = (a:Person {name:'a'})-[:KNOWS]-(b:Person {name:'b'}) return path ``` This new team member don't know that when I created this graph I considered that `:KNOWS` relationship is not directed. The result that he will see is the same: By the result this new team member can think that only Person a consider knows Person b. It seems to me bad. Not for you? This make any sense?