Nesting objects within other objects (in YAML)

[micsport13]

First, if this is not the best place to put this kind of question, please let me know so I can put this somewhere else.
I have been trying to serialize this class using a custom serializer and I have been having some trouble being able to get it serialize correctly.
Here's the class I'm trying to serialize:

public class Schema {
  @JsonProperty("table_constraints")
  @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY)
  private final Map<Constraint, Set<Column<?>>> constraints;

  private final Set<Column<?>> columns;

And here is the custom serializer:

public class SchemaSerializer extends StdSerializer<Schema> {

  @Override
  public void serialize( 
      Schema schema, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
      throws IOException {
    log.info("Serializing schema: " + schema.toString());
    jsonGenerator.writeStartObject();
    jsonGenerator.writeArrayFieldStart("columns");
    for (var column : schema.getColumns()) {
      jsonGenerator.writeObject(column);
    }
    jsonGenerator.writeEndArray();
    jsonGenerator.writeArrayFieldStart("constraints");
    for (var constraint : schema.getConstraints().entrySet()) {
      jsonGenerator.writeStartObject();
      jsonGenerator.writeObject(constraint.getKey())  <--- This line is the problem
      jsonGenerator.writeArrayFieldStart("columns");
      for (Column<?> column : constraint.getValue()) {
        jsonGenerator.writeString(column.getName());
      }
      jsonGenerator.writeEndArray();
      jsonGenerator.writeEndObject();
    }
    jsonGenerator.writeEndArray();
    jsonGenerator.writeEndObject();
  }
}

Here's the desired output:

 columns:
   - name: intColumn
      type:
        name: integer
     - name: varcharColumn
        type:
          name: varchar
          max_length: 1
constraints:
  - type: primary_key
    columns:
      - intColumn
  - type: unique
    columns:
      - varcharColumn

The constraint object is handled through annotations so I wasn't sure how to get it to just serialize the name provided in the annotation. If I try jsonGenerator.writeObjectField("type", constraint.getKey()), it wraps it with another "type" key instead of just providing the value. Any idea how to write another object within the same JsonObject?

[cowtowncoder]

Going forward, please mention which format you are talking about: looking at output, I assume it is YAML.

But beyond that, this sounds like an excellent Question for Discussions; I hadn't enabled Discussions yet for this repo, just did. Will convert.

[cowtowncoder]

As to whether you can write multiple values at same level in output hierarchy: no, every JsonSerializer is responsible for writing one and exactly one legal JSON Value (which may be Object, Array, Number, String, Boolean or null).
(apologies if I misunderstood the question)

[micsport13]

That makes sense. Based on my desired output, I was thinking that it was just one JSON object but since the Constraint is managed by annotations, it seems the JsonSerializer writes the objects separately. In this particular case, I was trying to get the object output of

{
type: primary_key, 
columns: ["intColumn"]
}

in JSON (I was talking about YAML in my question though but figured the JSON would be clearer of my intention). However when writing this object, it converts it into

{
type: primary_key
}, 
{
columns: ["intColumn"]
}

So my question is, is there a way to leave the object to be serialized by the annotation definition and embed it within another object?

[cowtowncoder]

Ok, so the logic relevant here is with jackson-databind, which is indeed format agnostic. But conversely this repo does not have much to do with it. That's ok for now, wrt Discussion.
I actually find JSON easier to read than YAML (wrt structure), for what that's worth.

You are correct in that use of EXTERNAL_PROPERTY does allow otherwise unusual outputting of value and type id as siblings. There are no other annotations that offer this that I can think of (although @JsonUnwrapped and @JsonAnyGetter do something similar).
But there may also be the reverse direction (deserialization to consider).

One other thing: this

jsonGenerator.writeObject(..)

may be problematic, depending on what you'd expect it to do. It is equivalent to ObjectMapper.writeValue().
If you just want to output JSON Object key, use JsonGenerator.writeFieldName().
All writeXxxField() methods are just convenience method over separate writeFieldName() + write[Valuetype]() methods; you don't need to use them for anything.
For JSON Arrays and Objects, you have pairs of writeStartXxx() / writeEndXxx() (where Xxx is Array or Object).