Using snakeCaseMappers, patching an object will not map inner keys, only the first level (columns), but using a FieldExpression will map the json field references.

class TestModel extends Model {
  ...
  static columnNameMappers = snakeCaseMappers();
  ...
}

// Query with array/object
TestModel
  .query()
  .patch({
    jsonColumn: [{
      innerKey: 1
    }]
  });

// Database output
{
  json_column: '[{"innerKey": 1}]' 
}

// Query with FieldExpression
TestModel
  .query()
  .patch({
    'jsonColumn:[0][innerKey]': 1
  });

// Database output
{
  json_column: '[{"inner_key": 1}]' 
}

EDIT: IMHO, this should work consistently across, and there could be an option for snakeCaseMappers | knexSnakeCaseMappers, something like deep|deepKeys|innerKeys. Obviously I would prefer it to default to false, and I'm guessing the usual expectation is around how the normal objects now work, but the default's not a big deal for me eitherway.