List value returned for a non-list uid predicate

[abhimanyusinghgaur]

What version of Dgraph are you using?

latest:master

Have you tried reproducing the issue with the latest release?

Yes

What is the hardware spec (RAM, OS)?

doesn't matter

Steps to reproduce the issue (command/config used to run Dgraph).

• Run dgraph zero
• Run dgraph alpha
• Set following dgraph schema:

uuid: string @index(hash) .
author: uid .
post: [uid] .

type Post {
   uuid
   author
}

type Author {
	uuid
	post
}

• That means, a Post can have only one Author, while an Author can have multiple Posts.
• Perform following mutation:

{
	"set": {
			"uid": "_:Post1",
			"uuid": "Post1",
			"dgraph.type": "Post",
			"author": {
				"uid": "_:Author1",
				"uuid": "Author1",
				"dgraph.type": "Author"
			}
		}
}

• Now, perform following query, to see what mutation did:

query {
    q1(func: eq(dgraph.type, "Post")) {
    	uid
    	uuid
    	author {
    		uid
    		uuid
    	}
    }
  }

• Result will be something like this (and that is fine):

{
    "data": {
        "q1": [
            {
                "uid": "0x2",
                "uuid": "Post1",
                "author": {
                    "uid": "0x1",
                    "uuid": "Author1"
                }
            }
        ]
    },
    "extensions": {
        "server_latency": {
            "parsing_ns": 68739,
            "processing_ns": 2970060,
            "encoding_ns": 19707,
            "assign_timestamp_ns": 600806,
            "total_ns": 3744032
        },
        "txn": {
            "start_ts": 12
        },
        "metrics": {
            "num_uids": {
                "author": 1,
                "dgraph.type": 0,
                "post": 1,
                "uid": 2,
                "uuid": 2
            }
        }
    }
}

• Let's do one more mutation, to set the author for Post1 to a new author:

{
	"set": {
			"uid": "0x2",
			"author": {
				"uid": "_:Author2",
				"uuid": "Author2",
				"dgraph.type": "Author"
			}
		}
}

• Again, perform the query to see results:

query {
    q1(func: eq(dgraph.type, "Post")) {
    	uid
    	uuid
    	author {
    		uid
    		uuid
    	}
    }
  }

Expected behaviour and actual result.

• Expected behaviour:

{
    "data": {
        "q1": [
            {
                "uid": "0x2",
                "uuid": "Post1",
                "author": {
                    "uid": "0x3",
                    "uuid": "Author2"
                }
            }
        ]
    },
    ...
}

• Actual Results:

{
    "data": {
        "q1": [
            {
                "uid": "0x2",
                "uuid": "Post1",
                "author": {
                    "uid": "0x1",
                    "uuid": "Author1",
                    "uid": "0x3",
                    "uuid": "Author2"
                }
            }
        ]
    },
    ...
}

Post1 should have Author2 as its only author, but it gives both Author1 and Author2 .

[abhimanyusinghgaur]

This is related to: https://discuss.dgraph.io/t/mutation-doesnt-update-value/6046

[martinmr]

ugh. So it looks like there used to be a check that would prevent you from adding multiple uids. This check was removed in 45a6d5b but the values are not being correctly overwritten. I think the code worked when I introduced this feature but it depended on this check and there was not a regression test so we didn't notice when the feature broke.

Two action items.

• Fix the bug
• Add a regression test.

[martinmr]

It looks like 1.2 and 1.2.1 are the only releases affected.