Merging columns and jsonb object array into a single jsonb column - sql

Question

I have a table visitors(id, email, first_seen, sessions, etc.)
and another table trackings(id, visitor_id, field, value) that stores custom, user supplied data.
I want to query these and merge the visitor data columns and the trackings into a single column called data
For example, say I have two trackings
(id: 3, visitor_id: 1, field: "orders_made", value: 2)
(id: 4, visitor_id: 1, field: "city", value: 'new york')
and a visitor
(id: 1, email: 'hello#gmail.com, sessions: 5)
I want the result to be on the form of
(id: 1, data: {email: 'hello#gmail.com', sessions: 5, orders_made: 2, city: 'new york'})
What's the best way to accomplish this using Postgres 9.4?

Answer 1

I'll start by saying trackings is a bad idea. If you don't have many things to track, just store json instead; that's what it's made for. If you have a lot of things to track, you'll become very unhappy with the performance of trackings over time.
First you need a json object from trackings:
-- WARNING: Behavior of this with duplicate field names is undefined!
SELECT json_object(array_agg(field), array_agg(value)) FROM trackings WHERE ...
Getting json for visitors is relatively easy:
SELECT row_to_json(email, sessions) FROM visitors WHERE ...;
I recommend you do not just squash all those together. What happens if you have a field called email? Instead:
SELECT row_to_json((SELECT
(
SELECT row_to_json(email, sessions) FROM visitors WHERE ...
) AS visitor
, (
SELECT json_object(array_agg(field), array_agg(value)) FROM trackings WHERE ...
) AS trackings
));