Blank Nodes and UID

Blank nodes in mutations, written _:identifier, identify nodes within a mutation. Dgraph creates a UID identifying each blank node and returns the created UIDs as the mutation result. For example, mutation:

{
 set {
    _:class <student> _:x .
    _:class <student> _:y .
    _:class <name> "awesome class" .
    _:class <dgraph.type> "Class" .
    _:x <name> "Alice" .
    _:x <dgraph.type> "Person" .
    _:x <dgraph.type> "Student" .
    _:x <planet> "Mars" .
    _:x <friend> _:y .
    _:y <name> "Bob" .
    _:y <dgraph.type> "Person" .
    _:y <dgraph.type> "Student" .
 }
}

results in output (the actual UIDs will be different on any run of this mutation)

{
  "data": {
    "code": "Success",
    "message": "Done",
    "uids": {
      "class": "0x2712",
      "x": "0x2713",
      "y": "0x2714"
    }
  }
}

The graph has thus been updated as if it had stored the triples

<0x6bc818dc89e78754> <student> <0xc3bcc578868b719d> .
<0x6bc818dc89e78754> <student> <0xb294fb8464357b0a> .
<0x6bc818dc89e78754> <name> "awesome class" .
<0x6bc818dc89e78754> <dgraph.type> "Class" .
<0xc3bcc578868b719d> <name> "Alice" .
<0xc3bcc578868b719d> <dgraph.type> "Person" .
<0xc3bcc578868b719d> <dgraph.type> "Student" .
<0xc3bcc578868b719d> <planet> "Mars" .
<0xc3bcc578868b719d> <friend> <0xb294fb8464357b0a> .
<0xb294fb8464357b0a> <name> "Bob" .
<0xb294fb8464357b0a> <dgraph.type> "Person" .
<0xb294fb8464357b0a> <dgraph.type> "Student" .

The blank node labels _:class, _:x and _:y do not identify the nodes after the mutation, and can be safely reused to identify new nodes in later mutations.

A later mutation can update the data for existing UIDs. For example, the following to add a new student to the class.

{
 set {
    <0x6bc818dc89e78754> <student> _:x .
    _:x <name> "Chris" .
    _:x <dgraph.type> "Person" .
    _:x <dgraph.type> "Student" .
 }
}

A query can also directly use UID.

{
 class(func: uid(0x6bc818dc89e78754)) {
  name
  student {
   name
   planet
   friend {
    name
   }
  }
 }
}