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Patterns to avoid when using JSON columns
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Rails supporting JSON field type is a great, but can screw you right over.
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Rails 5.2 introduced support for JSON field types, which is super awesome. It allows a really nice way of storing blobs of JSON against an object in your database. But while they're convenient, they can end up being a double edged sword and can lead to a headache if you aren't careful.

Here are two patterns I've seen popping up, which I think should be avoided.

Merging JSON data

Imagine a scenario where you'd like to store some adhoc data against a model, so you merge the new data over the old data e.g:

model = Model.find(params[:id])

# Set some values:
new_values = { important_value: :value }

# But in another HTTP request, at the same time you're running:
new_values = { other_important: :value }

model.jsonb_field.merge!(new_values)
model.save
model.jsonb_field['important_value'] # Could be empty or out of date.

This is a really risky pattern, because in a multi-threaded environments (e.g. two HTTP requests updating the same object around the same time) you'll run the risk of losing data. This is because from the point .find is called, the model may have been updated from another request.

A better approach is to store data you really want to be there in its own field. Most frameworks are smart enough to only update the fields you've changed (Rails included).

A replacement for columns

A pattern I've seen pop up, is the JSON columns being used as an alternative to putting things in their own columns, e.g:

user.settings = {
  dark_mode: true,
  send_weekly_digest: true,
  locale: 'en'
}

This is usually done in an attempt to normalise data to avoid N+1 queries, which is good & gems such as activerecord-typedstore make this process fairly manageable.

However, I've found querying against this data can be unreliable, e.g.

 User.where("settings @> ?", { dark_mode: true }.to_json) }

If the event the User model hadn't been saved since a new field in the JSON was added, this could return in incorrect number.

A better approach would be to add a migration for a field called settings_dark_mode with a default of true or false. While visually not as lovely, it does make for more reliable querying.

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