scope = Item.all

if params[:purpose]
  scope = scope.where(purpose: params[:purpose])
end

# и т.д.

scope = scope.where(title: "title ILIKE ?", params[:title]) if params[:title].present?

%i(title name description).select { |x| params[x].present? }.each do |p|
  scope = scope.where(p: "#{p} ILIKE ?", params[p])
end

user: {
  name: 'vasya',
  organization: {
    title: 'Hoofs & Horns, Ltd.',
    inn: 12345
  }
}

errors: {
  user: {
    organization: {
      inn: ["must be a number"]
    }
  }
}