val function in upsert

The upsert block allows performing queries and mutations in a single request. The upsert block contains one query block and one or more than one mutation blocks. Variables defined in the query block can be used in the mutation blocks using the uid and val function.

The val function allows extracting values from value variables. Value variables store a mapping from UIDs to their corresponding values. Hence, val(v) is replaced by the value stored in the mapping for the UID (Subject) in the N-Quad. If the variable v has no value for a given UID, the mutation is silently ignored. The val function can be used with the result of aggregate variables as well, in which case, all the UIDs in the mutation would be updated with the aggregate value.

Example of val Function

Let’s say we want to migrate the predicate age to other. We can do this using the following mutation:

curl -H "Content-Type: application/rdf" -X POST localhost:8080/mutate?commitNow=true -d $'
upsert {
  query {
    v as var(func: has(age)) {
      a as age
    }
  }

  mutation {
    # we copy the values from the old predicate
    set {
      uid(v) <other> val(a) .
    }

    # and we delete the old predicate
    delete {
      uid(v) <age> * .
    }
  }
}' | jq

Result:

{
  "data": {
    "code": "Success",
    "message": "Done",
    "uids": {}
  },
  "extensions": {...}
}

Here, variable a will store a mapping from all the UIDs to their age. The mutation block then stores the corresponding value of age for each UID in the other predicate and deletes the age predicate.

We can achieve the same result using json dataset as follows:

curl -H "Content-Type: application/json" -X POST localhost:8080/mutate?commitNow=true -d $'{
  "query": "{ v as var(func: regexp(email, /.*@company1.io$/)) }",
  "delete": {
    "uid": "uid(v)",
    "age": null
  },
  "set": {
    "uid": "uid(v)",
    "other": "val(a)"
  }
}' | jq