Date and 'polymorphic' class causes JsonGenerationException: BsonSerializer can only be used with BsonGenerator

Question

When updating to jongo 1.3.0 we started to get the following error when reading documents from MongoDB:

com.fasterxml.jackson.core.JsonGenerationException: BsonSerializer can only be used with BsonGenerator

After some testing I found that the problem occurs when using @JsonTypeInfo and the MongoDB document contains a date object before the type property. Given:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.EXISTING_PROPERTY,
        property = 'type',
        visible = true)
@JsonSubTypes([
        @JsonSubTypes.Type(name = 'a', value = A),
        @JsonSubTypes.Type(name = 'b', value = B)
])
abstract class Base {
    String string // For reference
    Date date
    String type
}

class A extends Base { A() { type = 'a' } }

class B extends Base { B() { type = 'b' } }

This (spock) test will fail

def mapper = new ObjectMapper(new BsonFactory()).registerModule(new BsonModule())
def bytes = mapper.writeValueAsBytes(original)

expect:
def parsed = mapper.readValue(bytes, Base)
parsed instanceof A // com.fasterxml.jackson.core.JsonGenerationException: BsonSerializer can only be used with BsonGenerator
parsed.string == original.string
parsed.date == original.date // parsed.date is null with 'de.undercouch:bson4jackson:2.8.0-SNAPSHOT'
parsed.type == original.type

where:
testCase                  | original
'A'                       | new A(string: 'string', date: new Date(), type: 'a') // fails
'String, Date, Type'      | [string: 'string', date: new Date(), type: 'a']      // fails
'String, null date, Type' | [string: 'string', date: null, type: 'a']
'String, Type, Date'      | [string: 'string', type: 'a', date: new Date()]
'Type, String, Date'      | [type: 'a', string: 'string', date: new Date()]

Note that if date is null or comes after type the test passes.

I would like to update Jongo and Jackson but don't think I can guarantie the order of properties on our DB. Question is if the issue can be solved.

• I've tried updating Jackson to 2.8.6 and 2.8.7 but no difference.
• When I try de.undercouch:bson4jackson:2.8.0-SNAPSHOT the parsed date will be null if it comes after type

A similar bug seems to have been solved with 2.8.0-SNAPSHOT: https://github.com/michel-kraemer/bson4jackson/issues/67

I've posted an issue here: https://github.com/michel-kraemer/bson4jackson/issues/72

Answer 1

Ensuring ordering should not be necessary, as content will be buffered as necessary. However, this did trigger a problem since while TokenBuffer implements base JsonGenerator, it does not (and can not) implement BSON-specific subtype.

But I think that in addition to upgrading jackson-databind and jackson-core to 2.8.6 (which makes sense; fix was in 2.8.3 or so) you will also need a newer version of bson4jackson. However it does not look like there is yet release of 2.8, only 2.7. Need to add a note on issue you referred to ask for a release.