Add support for conditional mutations

[srfrog]

This PR adds initial API support for conditional mutations (txn block).

New mutation signature:

txn {
  # conditional query that yields a var(s)
  query {
    # ...
  }
  # mutation that depends on query above and may use var values
  mutation {
    # ...
  }
}

Breaking changes:

• New API field cond_query in Mutation gRPC message for sending conditional query.
• New API field subject_var in NQuad gRPC message to indicate variable name instead of subject uid
• New API field object_var in NQuad gRPC message to indicate variable name instead of object id

An example of proper usage is also included.

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This change is 

[martinmr]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @srfrog)

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example_mutation_txn_test.go, line 1 at r1 (raw file):

/*

Maybe move this file and the rest of the examples to a separate directory?

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protos/api.proto, line 134 at r1 (raw file):

Value var_name = 8;

How is this field supposed to be used? I do not see it in the example.

[srfrog]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @martinmr)

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example_mutation_txn_test.go, line 1 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Maybe move this file and the rest of the examples to a separate directory?

Sure, but maybe not for this particular PR.

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protos/api.proto, line 134 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Value var_name = 8;

How is this field supposed to be used? I do not see it in the example.

VarName has the name of the query variable for this nquad. When assigning uids before mutation, we use the value from the variable to alias the subject. Similar to _: but the inverse.

See https://github.com/dgraph-io/dgraph/pull/3197/files#diff-e9752ad411f442cb414981895be0a228R153

[gitlw]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @martinmr)

[martinmr]

Your examples are being run right now by the integration tests and failing. I would either disable the tests for now or revert them and re-add them once they can pass. It's fine if this PR only includes the API changes.

[srfrog]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @martinmr)

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example_mutation_txn_test.go, line 1 at r1 (raw file):

Previously, srfrog (Gus) wrote…

Sure, but maybe not for this particular PR.

Done.

[gitlw]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @martinmr)

[martinmr]

Reviewed 3 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved