Add foreign_key_exists? method.

[estum]

No description provided.

[senny]

@estum thank you for your work. Would you mind sharing some use-cases where you can see this being useful?

[estum]

@senny I was missing a method like index_exists? (which useful for me too) while reorganizing migrations to avoid add_foreign_key``remove_foreign_key methods raise when a key is already created\removed. I was surprised it yet hasn't been implemented.

[ianks]

We should probably return false instead of nil here to maintain convention with most Ruby .exist? functions.

[senny]

@ianks that's not necessary. Rails does not make the promise of returning real booleans from predicate methods.

[ianks]

Is there a particular reason for now? I would consider it an anti-pattern to return nil from a predicate method as it makes it harder to reason about the function.

[senny]

@ianks there is a section on booleans in our API documentation guidelines.

[senny]

@estum great work! 💛

I added a CHANGELOG entry and made some minor changes in the merge commit (removed trailing whitespace, replaced all?{|assoc|} with all? {|key, value|} and got rid of try(:name).

The extraction of foreign_key_for already paid off. Merging this I found a bug in a recent patch I committed (8980da8).

[claudiob]

@senny Is this # :nodoc: intentional? Or should we make this method publicly documented?

[senny]

@claudiob I think it was due to the fact that foreign_key had a no-doc as well. It was previously only used by references. Should be fine to document them though.