[AVRO-2730] Union: validating default values

• Type: Wish
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Component/s: spec
• Assignee: Unassigned
• Reporter: Andy Le
• Created: 07/Feb/20 04:30
• Updated: 07/Feb/20 15:00
• Resolved: 07/Feb/20 15:00

Description

Hi guys,

I'm trying to set the default value for a field (using Java) and I've got a nasty exception:

Exception in thread "main" org.apache.avro.AvroTypeException: Invalid default for field first: "Andy" not a ["null","string"]

    at org.apache.avro.Schema.validateDefault(Schema.java:1542)
...

In the documentation of Apache Avro: Unions

Unions, as mentioned above, are represented using JSON arrays. For example, ["null", "string"] declares a schema which may be either a null or string.

(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.)

Would you please tell me why the type of the default value must match the first element of the union? Is it better if the value match any element?