Add NullSerializer for JSON database columns

[bdewater]

I've been using this in my application using Postgres' JSONB data type for the data column and it is working well for my needs. For example I can use something like the code below to get a list of sessions so the user can select one to log out:

Session.where(%(data @> '{"user_id": ?}'), current_user.id)

I'm unsure what the best way is to go about adding tests for this - do we need to add PostgreSQL/MySQL to the test suite?

[rails-bot]

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