将可为空的项目写入Avro C中的avro记录 [英] Write nullable item to avro record in Avro C

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

(请注意，当为类型为联合的记录字段指定默认值时，默认值的类型必须与联合的第一个元素匹配.因此，对于包含"null"的联合，"null"通常会首先列出，因为此类联合的默认值通常为null.)

(Note that when a default value is specified for a record field whose type is a union, the type of the default value must match the first element of the union. Thus, for unions containing "null", the "null" is usually listed first, since the default value of such unions is typically null.)

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