Is there a way to fix this ugliness

[Tim Uckun <timuckun@xxxxxxxxx>]

I am trying to get the child elements of a one to many table to be rolled up into a json field in the parent table. The query I am running is 

select 

    ob.id

     ,case when array_position(array_agg(im.image_type), null) = 1  then '[]' else      json_agg(row_to_json(im.*)) end as images

     from observations ob

   left join images im on ob.id = im.observation_id

group by 1

The reason I have the case statement there is because some observations don't have images but the json_agg(row_to_json function returns [NULL] instead of [] which is what I really want.

Is there a more elegant way to do this?

I searched on the internet and somebody suggested coalesce but no matter what combination I tried I could not make it happen.

Thanks.