Fix nil FK queries, strict_where validation, field name conflict warnings

[sawirricardo]

Summary

Three low-hanging fixes addressing open issues.

Fixes #302 — belongs_to with nil foreign key makes useless query

When accessing a belongs_to association with a nil foreign key, ActiveHash was still calling find_by_* which triggers a DB query. ActiveRecord skips this query when the key is nil.

Fix: Early return nil when foreign key is nil in:

• ActiveRecordExtensions#belongs_to
• belongs_to_active_hash
• Methods#belongs_to
• Methods#has_one

No breaking change — nil key previously returned nil anyway after a useless query.

Fixes #190 — where(undefined_key: nil) returns all records

where() with an undefined field silently returns all records because read_attribute returns nil for unknown keys, and nil == nil matches everything.

Fix: Opt-in strict_where class attribute. When enabled:

class Country < ActiveHash::Base
  self.strict_where = true
  fields :name, :code
end

Country.where(typo_key: nil)
# => ArgumentError: Unknown field(s): typo_key. Valid fields are: id, name, code

Also validates against data keys for models that use undeclared fields via data=. Opt-in only — no breaking change.

Fixes #184 — field :display silently returns nil

Rails adds display to Object. ActiveHash skips defining the method if one already exists, but gives no indication. Users get silent nil returns and confusing test failures.

Fix: warn() in validate_field when field name conflicts with an existing method:

class TestHash < ActiveHash::Base
  field :display
  # => warning: ActiveHash: Field :display on TestHash conflicts with an existing method...
end

Additive warning only — no breaking change.

Test Results

276 specs pass, 2 pre-existing sqlite3 adapter failures unrelated to these changes.

[flavorjones]

@sawirricardo Thanks very much for opening this PR!

However, if this is three individual fixes, we would prefer it to be broken out into three separate pull requests, so each change can be reviewed and merged on its own.

Also, I would like to see tests added that clearly describe the problem being fixed. If you need any help figuring out to craft an appropriate test, I'm happy to help -- just drop a note here!

I appreciate your time -- thanks for understanding how we like to work. I hope you're able to make time to contribute in the future! ❤️