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Enumerable#pluck has just landed on master, providing similar functionality as Active Record's #pluck.

This allows for something like
[{ name: "David" }, { name: "Rafael" }, { name: "Aaron" }].pluck(:name) to get everyone's names. Pretty handy!

Active Record now supports specifying a collation function for SQLite string columns.

With the new Attributes API, it is now possible to specify a proc as the default values for an attribute, which allows for runtime-generated default values such as attribute :year, :integer, default: -> { Time.now.year }. See also these related changes.

By default, Rails overrides MySQL's sql_mode setting with sql_mode=STRICT_ALL_TABLES to prevent silent data loss. While this is helpful, it has the unfortunate side-effect of overriding any user customizations.

If you prefer to manage this yourself, you can now specify strict: :default in your connection's config and Rails will leave your sql_mode alone. (STRICT_ALL_TABLES is still highly recommended though!)

If you are using render nothing: true, you should start replacing it with head :ok, which does the same thing and is more semantic. This option will go away permanently in Rails 5.1.

The superficial similarity between Relation#uniq and Array#uniq has been a source of confusion, which led to the addition of Relation#distinct which better communicates what is happening under the hood.

The recent discussion on #20198 put the final nail in the coffin for Relation#uniq and its friends, #uniq! and #uniq_value. These methods are deprecated and scheduled for removal in Rails 5.1, so you might want to start using #distinct, #distinct! and #distinct_value soon.