filters on one-to-many relationship existence

[nmonterroso]

Similar to neo4j-graphql-java#27.

Is it possible to use filter that says "where an array field is non-empty"? For example this schema:

type Person {
	name: String!
	friends: [Person!]! @relation(name:"knows",direction:OUT)
	pets: [Pet!] @relation(name:"knows",direction:OUT)
}

type Pet {
	name: String!
}

on a db with:

create(n1:Person {name:"foo"}),
	(n2:Person {name:"bar"}),
	(n3:Pet {name:"baz"}),
	(n1)-[:knows]->(n2),
	(n2)-[:knows]->(n3)

I'm struggling to come up with a filter that only returns the Persons that have at least one [:knows]-(:Person) relationship. Based on the generated cypher It seems like friends_not: null is the intended way to do this:

MATCH (`person`:`Person`)
WHERE ( (`person`)-[:`knows`]->())
RETURN graphql.labels(`person`) AS `_labels`,
`person`.`name` AS `name`,
[ (`person`)-[:`knows`]->(`person_friends`:`Person`)  | `person_friends` {`_labels` : graphql.labels(`person_friends`), .`name`}] AS `friends`,
[ (`person`)-[:`knows`]->(`person_pets`:`Pet`)  | `person_pets` {`_labels` : graphql.labels(`person_pets`), .`name`}] AS `pets`

The key being WHERE ( (person)-[:knows]->()) but since the second node doesn't have a label the following query:

{
  Person (filter: { friends_not: null}) {
    name
    friends {
      name
    }
    pets {
      name
    }
  }
}

gives me:

    "Person": [
      {
        "name": "foo",
        "friends": [
          {
            "name": "bar"
          }
        ],
        "pets": []
      },
      {
        "name": "bar",
        "friends": [],
        "pets": [
          {
            "name": "baz"
          }
        ]
      }
    ]

[jexp]

Can you try to use:

friends_some : { name_not: null }

Currently there is no generic, "does exist"
But it's a good idea to add.