Add validator for integer arrays

Example usage:

    validates :related_post_ids, integer_array: true

app/validators/integer_array_validator.rb

@@ -0,0 +1,63 @@
+# Validates an integer array
+#
+# This uses Integer() behind the scenes.
+#
+# === Example
+#
+#   class Post
+#     include ActiveModel::Validations
+#
+#     attr_accessor :related_post_ids
+#     validates :related_post_ids, integer_array: true
+#   end
+#
+#   post = Post.new
+#
+#   post.related_post_ids = nil
+#   post.valid?                     # => true
+#
+#   post.related_post_ids = []
+#   post.valid?                     # => true
+#
+#   post.related_post_ids = 1
+#   post.valid?                     # => false
+#   post.errors[:related_post_ids]  # => ["must be an array"]
+#
+#   post.related_post_ids = [1, 2, 3]
+#   post.valid?                     # => true
+#
+#   post.related_post_ids = ["1", "2", "3"]
+#   post.valid?                     # => true
+#
+#   post.related_post_ids = [1, "2", "Not Integer", 3]
+#   post.valid?                     # => false
+#   post.errors[:related_post_ids]  # => ["must contain only valid integers"]
+class IntegerArrayValidator < ActiveModel::EachValidator
+  NOT_ARRAY_ERROR = I18n.t("validators.integer_array_validator.not_array_error")
+  INVALID_ELEMENT_ERROR = I18n.t("validators.integer_array_validator.invalid_element_error")
+
+  def validate_each(record, attribute, value)
+    return if value.nil?
+
+    validate_attribute_is_array(record, attribute, value)
+    validate_attribute_elements_are_integer(record, attribute, value)
+  end
+
+  protected
+
+  def validate_attribute_is_array(record, attribute, value)
+    return if value.is_a?(Array)
+
+    record.errors.add(attribute, NOT_ARRAY_ERROR)
+  end
+
+  def validate_attribute_elements_are_integer(record, attribute, array)
+    return unless array.is_a?(Array)
+
+    array.each do |element|
+      Integer(element)
+    end
+  rescue ArgumentError
+    record.errors.add(attribute, INVALID_ELEMENT_ERROR)
+  end
+end