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
}
}
}
}
This is a companion discussion topic for the original entry at https://dgraph.io/docs/mutations/blank-nodes/