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From "Alexandre Normand (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AVRO-1145) Can't read union of null and primitive from value written with schema as primitive
Date Fri, 31 Aug 2012 18:31:07 GMT

    [ https://issues.apache.org/jira/browse/AVRO-1145?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13446211#comment-13446211
] 

Alexandre Normand commented on AVRO-1145:
-----------------------------------------

The documentation in [parsing.html|http://svn.apache.org/viewvc/avro/trunk/lang/java/avro/src/main/java/org/apache/avro/io/parsing/doc-files/parsing.html?view=log]
seems to suggest that we should have a {{ReaderUnionAction}} to handle this case. I naively
expected to find an action class called {{ReaderUnionAction}} much in the same way there is
a {{WriterUnionAction}}. 

Is that something that's simply missing from the current implementation?
                
> Can't read union of null and primitive from value written with schema as primitive
> ----------------------------------------------------------------------------------
>
>                 Key: AVRO-1145
>                 URL: https://issues.apache.org/jira/browse/AVRO-1145
>             Project: Avro
>          Issue Type: Bug
>          Components: java
>    Affects Versions: 1.7.0
>            Reporter: Alexandre Normand
>         Attachments: TestPrimitiveToUnionResolution.java
>
>
> Using the its Java's generic representation API and I have a problem dealing with our
current case of schema evolution. The scenario we're dealing with here is making a primitive-type
field optional by changing the field to be a {{union}} of {{null}} and that primitive type.
> I'm going to use a simple example. Basically, our schemas are:
> Initial: A record with one field of type {{int}}
> Second version: Same record, same field name but the type is now a union of {{null}}
and {{int}}
> According to the [schema resolution|http://avro.apache.org/docs/current/spec.html#Schema+Resolution]
chapter of Avro's spec, the resolution for such a case should be:
> {code}
> if reader's is a union, but writer's is not
> The first schema in the reader's union that matches
> the writer's schema is recursively resolved against 
> it. If none match, an error is signalled.
> {code}
> My interpretation is that we should resolve data serialized with the initial schema properly
as int is part of the union in the reader's schema.
> However, when running a test of reading back a record serialized with version 1 using
the version 2, I get
> *{{org.apache.avro.AvroTypeException: Attempt to process a int when a union was expected.}}*
> Here's a test that shows exactly this:
> {code}
> @Test
> public void testReadingUnionFromValueWrittenAsPrimitive() throws Exception {
>     Schema writerSchema = new Schema.Parser().parse("{\n" +
>             "    \"type\":\"record\",\n" +
>             "    \"name\":\"NeighborComparisons\",\n" +
>             "    \"fields\": [\n" +
>             "      {\"name\": \"test\",\n" +
>             "      \"type\": \"int\" }]} ");
>     Schema readersSchema = new Schema.Parser().parse(" {\n" +
>             "    \"type\":\"record\",\n" +
>             "    \"name\":\"NeighborComparisons\",\n" +
>             "    \"fields\": [ {\n" +
>             "      \"name\": \"test\",\n" +
>             "      \"type\": [\"null\", \"int\"],\n" +
>             "      \"default\": null } ]  }");
>     // Writing a record using the initial schema with the 
>     // test field defined as an int
>     GenericData.Record record = new GenericData.Record(writerSchema);
>     record.put("test", Integer.valueOf(10));        
>     ByteArrayOutputStream output = new ByteArrayOutputStream();
>     JsonEncoder jsonEncoder = EncoderFactory.get().
>        jsonEncoder(writerSchema, output);
>     GenericDatumWriter<GenericData.Record> writer = new 
>        GenericDatumWriter<GenericData.Record>(writerSchema);
>     writer.write(record, jsonEncoder);
>     jsonEncoder.flush();
>     output.flush();
>     System.out.println(output.toString());
>     // We try reading it back using the second schema 
>     // version where the test field is defined as a union of null and int
>     JsonDecoder jsonDecoder = DecoderFactory.get().
>         jsonDecoder(readersSchema, output.toString());
>     GenericDatumReader<GenericData.Record> reader =
>             new GenericDatumReader<GenericData.Record>(writerSchema, 
>                 readersSchema);
>     GenericData.Record read = reader.read(null, jsonDecoder);
>     // We should be able to assert that the value is 10 but it
>     // fails on reading the record before getting here
>     assertEquals(10, read.get("test"));
> }
> {code}

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