Mark Rails/UniqBeforePluck cops as unsafe

[kunitoo]

Rails/UniqBeforePluck is unsafe depending on the database collation.
Here is an example of an unsafe case for the three databases that Rails supports(MySQL, SQLite, PostgreSQL).

In the case of MySQL

The default collation is utf8mb4_general_ci(in Rails), so it is unsafe.

An example is as follows.

• Create table

class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name
    end
  end
end

• Create data

User.create!(name: 'A')
User.create!(name: 'a')

before autocorrect

User.pluck(:name).uniq #=> ["A", "a"]

after autocorrect

User.distinct.pluck(:name) #=> ["A"]

In the case of SQLite

The default collation is BINARY, so it is safe.
If you change the collation to NOCASE, the collation is unsafe.

The following is an example.

• Create table

class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name, collation: 'NOCASE'
    end
  end
end

Other than the above, it is the same as MySQL.

In the case of PostgreSQL

The default collation is safe.
If you create a custom collation and specify it, the collation will be unsafe.

A concrete example is as follows.

• Create custom collation

CREATE COLLATION case_insensitive (
  provider = icu,
  locale = 'und-u-ks-level2',
  deterministic = false
);

• Create table

class CreateUsers < ActiveRecord::Migration[6.0]
  def change
    create_table :users do |t|
      t.string :name, collation: 'case_insensitive'
    end
  end
end

Other than the above, it is the same as MySQL.

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Good catch! Thank you for the great investigation and explanation!