Fixes #XXX where filtering by scopes that accept arrays as single arguments would raise ArgumentError: wrong number of arguments (given 2, expected 1).

Problem

When using scopes with arity 1 that expect an array as a single parameter, Ransack would incorrectly splat the array arguments, causing argument count mismatches:

class Person < ActiveRecord::Base
  scope :domestic, ->(countries) { where(country: countries) }
  
  def self.ransackable_scopes(auth_object = nil)
    [:domestic]
  end
end

# This would raise ArgumentError
Person.search(domestic: ['US', 'JP'])
# ArgumentError: wrong number of arguments (given 2, expected 1)

# Users had to work around this with nested arrays
Person.search(domestic: [['US', 'JP']])  # Workaround

Root Cause

The chain_scope method in lib/ransack/context.rb was always using the splat operator (*args) when calling scopes, regardless of the scope's arity. For scopes expecting a single array argument, this would split ['US', 'JP'] into two separate arguments 'US', 'JP', violating the lambda's arity constraint.

Solution

Added special handling in chain_scope to detect when a scope has arity 1 and receives an array argument. In this case, the array is passed as a single argument instead of being splatted:

def chain_scope(scope, args)
  return unless @klass.method(scope) && args != false
  @object = if scope_arity(scope) < 1 && args == true
              @object.public_send(scope)
            elsif scope_arity(scope) == 1 && args.is_a?(Array)
              # For scopes with arity 1, pass the array as a single argument
              @object.public_send(scope, args)
            else
              @object.public_send(scope, *args)
            end
end

Testing

Added comprehensive test coverage for various scope arity scenarios:

• Arity 1 scopes: Arrays are now passed as single arguments (main fix)
• Flexible arity scopes (negative arity): Arrays continue to be splatted as expected
• Multi-parameter scopes (arity > 1): Arrays continue to be splatted as expected
• Backwards compatibility: The nested array workaround continues to work

Impact

• ✅ Fixes the reported issue: Person.search(domestic: ['US', 'JP']) now works
• ✅ No breaking changes: All existing functionality preserved
• ✅ Backwards compatible: Existing workarounds continue to work
• ✅ Minimal change: Only 3 lines of code added for surgical fix

This change makes Ransack's scope handling more intuitive and eliminates the need for users to understand lambda arity details when using array arguments with scopes.

Fixes #404

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