[jira] [Commented] (AVRO-3946) Support single-element unions for Java code generation

["Thiruvalluvan M. G. (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-3946?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17819890#comment-17819890 ] 

Thiruvalluvan M. G. commented on AVRO-3946:
-------------------------------------------

Interesting. We have two options:
 # Fix the specification and disallow single-element unions, just like Haskell disallows single-element tuples, for example. Or,
 # As you suggest, continue to allow single-element union and treat it equivalent to the standalone element type.

While (1) will be a breaking change, (2) will promote multiple representations of the schema, which is not desirable. But then, we already have multiple representations: {{{{}"name": myField", "type": "long"{}}}} and {{{{}"name": myField", "type": {"type": "long"{}}}}}. So, (2) does not introduce any new issue.

+1 for your proposal.

> Support single-element unions for Java code generation
> ------------------------------------------------------
>
>                 Key: AVRO-3946
>                 URL: https://issues.apache.org/jira/browse/AVRO-3946
>             Project: Apache Avro
>          Issue Type: Improvement
>          Components: java, tools
>    Affects Versions: 1.11.3
>         Environment: [Avro specification|https://avro.apache.org/docs/1.11.1/specification/#unions]
>            Reporter: M. Justin
>            Priority: Minor
>
> I am writing code to integrate with existing Avro-based data created by someone else.  Their schema currently has one-element unions for their types, rather than just the specific types.  For example:
> {code:json}
> {
>   "type": "record",
>   "name": "Issue",
>   "namespace": "invalid.example",
>   "fields": [
>     {
>       "name": "myField",
>       "type": ["long"]
>     }
>   ]
> }
> {code}
> instead of:
> {code:json}
> {
>   "type": "record",
>   "name": "Issue",
>   "namespace": "invalid.example",
>   "fields": [
>     {
>       "name": "myField",
>       "type": "long"
>     }
>   ]
> }
> {code}
> While this isn't idiomatic, looking at the [Avro Specification|https://avro.apache.org/docs/1.11.1/specification/#unions], it does appear to be valid.  It should also be logically equivalent to the non-union version of this.
> The Java code generator, however, generates this field as an {{Object}}, not as a {{long}} like it would with the non-union type (or the {{Long}} if it were the {{["null", "long"]}} or {{["long", "null"]}} union).
> {code}
> java -jar avro-tools-1.11.3.jar compile schema issue.avsc .
> {code}
> produces
> {code:java}
>   private java.lang.Object myField;
> {code}
> instead of
> {code:java}
>   private long myField;
> {code}
> For my case, I'm going to work with the other project's developers to change their schema type to be the primitive type.
> However, I'm raising this issue in case you want to improve the handling of this case to have a single-element union generate the same code as it would if just as if it were instead just that single element.  i.e. {{\{"name": myField", "type": ["long"]\}}} would generate the same code as {{\{"name": myField", "type": "long"\}}} currently does, which is as a {{long}} value in the Java code.



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