How do I check if a json key exists in Postgres?

You can also use the ‘?’ operator like that:

SELECT '{"key_a":1}'::jsonb ? 'key_a'

And if you need to query by nested key, use like this:

SELECT '{"key_a": {"nested_key": "a"}}'::jsonb -> 'key_a' ? 'nested_key' 

See http://www.postgresql.org/docs/9.5/static/functions-json.html

NOTE: Only for jsonb type.