Fix false positive for UniqueValidationWithoutIndex when using conditions

[sunny]

Hey everybody!

This fixes a false positive for Rails/UniqueValidationWithoutIndex when using conditions, such as:

class User < ApplicationRecord
  validates :email, uniqueness: true, unless: :guest?
end

---

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[koic]

@pocke Can you take a look at this one?

[koic]

Hm, I think this is not a false positive because email address is expected to be unique regardless of conditions.

[sunny]

Hi @koic! Thanks for taking the time to review this.

Depending on your application, you may have certain fields that need to validate uniqueness under certain conditions.

Here email does not have a uniqueness validation because it allows blank values in some conditions : the account has created without an email or the user has been anonymised.

Here's another example taken from my application (cults3d.com):

 validates :nick,
           presence: true,
           uniqueness: true,
           length: { minimum: 3 },
           exclusion: { in: ReservedWords.all },
           if: :registered?

In this example the nick is only tested for uniqueness (and other validators) if the user is registered. Until the user is considered registered, the model can have a blank (and therefore non-unique) email.

In both cases we cannot a unique index to these fields, since it would break the application.

[koic]

Thank you for showing a specific use case. Perhaps it would be better to make it configurable, but I think this change is acceptable.