FAQ

[Avro-user] Using optional fields (nullable unions) with the C-API

Douglas Creager
Nov 22, 2011 at 1:51 pm

I'm using a record like this

{
"type" : "record",
"name" : "TestPacket",
"fields" : [
{ "name" : "Id" , "type" : "int"},
{ "name" : "Value" , "type" : ["string","null"]}
]
}

But if I just not write anything to the 'Value' filed,
avro_writer_memory() refuses to validate it. How do you normally use the
optional fields with the C-API?
Hi Markus

In Avro, an “optional” field is really just a common pattern where you've got a union, with a “null” for one branch, and some other type for the second branch. And with all union types, you have to explicitly tell the Avro library which branch is active for a particular value. (Think of the union as a container that must always contain exactly one value, but where the type of that value can change.)

So, if you're using the new value API, you'd use something like:

avro_value_t rec = /* given */;
avro_value_t field, branch;
avro_value_get_by_name(rec, "Value", &field, NULL);
avro_value_set_branch(&field, 1, &branch);
avro_value_set_null(&branch);

and if you're using the old datum API:

avro_datum_t rec = /* given */;
avro_datum_t field, branch;
avro_record_get(rec, "Value", &field);
avro_union_set_discriminant(field, 1, &branch);
/* no avro_null_set in the old API */

In set_branch/set_discriminant, the 1 refers to the index of the "null" type in the union. I tend to use branch 0 for the null type in these kinds of unions, but that's just a matter of aesthetics.

cheers
–doug
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Markus Resch: 1 post Douglas Creager: 1 post