The correct JSON syntax for Dgraph would be
[
{
"uid": "_:30",
"ToNodeId": [ { "uid": "_:1412" } ]
},
...
]
Note that you have to add the prefix _: in order to make them “unique” via blank node. Remember that Blank Nodes guarantees the uniqueness only in the procedure context(or transaction context) you can’t use it for future insertions/mutations.
It may possible to use numbers or hex, but you have to lease several uids from zero before doing it.