将可为空的项目写入Avro C中的avro记录

问题描述

模式:

const char schema[] =
    "{ \"type\":\"record\", \"name\":\"foo\","
    "\"fields\": ["
        "{ \"name\": \"nullableint\", \"type\":[\"int\",\"null\"]}"
    "]}";

设置架构:

avro_datum_t foo_record = avro_record(schema);

设置可为空的原点:

avro_datum_t nullableint = avro_int32(1);

设置项目:

int err = avro_record_set(foo_record,"nullableint",nullableint);

写项目:

int err2 = avro_file_writer_append(avro_writer, foo_record);

---

并且有一个错误.不知何故，我必须设置可为空的条目的分支，但是我看不到有任何函数可以做到这一点.

---

And there is an error. Somehow, I must set the branch of my nullable entry, yet I see no functions that will do this.

如何将此值设置为 null 或int?

How do I set this value to either null or an int?

推荐答案

请参阅以下代码，该代码设置并集分支和int分支:

See the following code which sets the union branch and the int branch:

注意:对于联合，您需要在设置数据之前设置相关分支.联合分支索引从0开始，称为判别式.对于本示例中的int字段，判别式为0(因为它是第一个字段).对于null分支，判别式为1(因为它是第二个字段).以null作为第一个分支更为常见，但是，我遵循了您的问题示例.

Note: For union, you need to set the relevant branch before you set the data. The union branch index starts from 0 and is called discriminant. For the int field at this example the discriminant is 0 (as it is the first field). For the null branch the discriminant is 1 (as it is the second field). It is more common having the null as the first branch, however, I followed your question example.

#include "avro.h"
void main()
{
    const char schema_json[] =
        "{ \"type\":\"record\", \"name\":\"foo\","
        "\"fields\": ["
        "{ \"name\": \"nullableint\", \"type\":[\"int\",\"null\"]}"
        "]}";
    avro_schema_t my_schema;
    avro_schema_from_json( schema_json, 0, &my_schema, NULL );
    avro_datum_t  my_record = avro_datum_from_schema( my_schema );
    avro_datum_t  my_int_field = NULL;
    avro_datum_t  branch = NULL;
    char  *json = NULL;

    //get the int field
    avro_record_get( my_record, "nullableint", &my_int_field );

    //set the int branch on this field
    avro_union_set_discriminant( my_int_field, 0, &branch );
    //set value of 100 at this int branch
    avro_int32_set( branch, 100 );
    //convert the datum record data to json
    avro_datum_to_json( my_record, 1, &json );
    printf( "got json for int branch: %s\n", json );

    //get the int field
    avro_record_get( my_record, "nullableint", &my_int_field );
    //set the null branch on this field
    avro_union_set_discriminant( my_int_field, 1, &branch );
    //convert the datum record data to json
    avro_datum_to_json( my_record, 1, &json );
    printf( "got json for null branch: %s\n", json );

}

我将数据打印为json，以便更好地检查结果.该程序的输出是

I printed the data as json for better examination of the results. output of this program is

got json for int branch: {"nullableint": {"int": 100}}
got json for null branch: {"nullableint": null}

顺便说一句，约定是将空分支作为第一个分支，具有

BTW, the convention is to have the null branch as the first branch, having

"{ \"name\": \"nullableint\", \"type\":[\"null\",\"int\"]}"

和不是

"{ \"name\": \"nullableint\", \"type\":[\"int\",\"null\"]}"

请参见avro规范中的以下引用: https://avro.apache.org/docs/1.8.2/spec.html

see this cite from avro specification: https://avro.apache.org/docs/1.8.2/spec.html

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