Add validator for datetime string

Example usage:

    validates :start_at, date_time_string: true

app/validators/date_time_string_validator.rb

@@ -0,0 +1,63 @@
+# Validates a datetime string with relaxed rules
+#
+# This uses ActiveSupport::TimeZone.parse behind the scenes.
+#
+# https://api.rubyonrails.org/classes/ActiveSupport/TimeZone.html#method-i-parse
+#
+# === Example
+#
+#   class Post
+#     include ActiveModel::Validations
+#
+#     attr_accessor :published_at
+#     validates :published_at, date_time_string: true
+#   end
+#
+#   post = Post.new
+#
+#   post.published_at = nil
+#   post.valid?                 # => true
+#
+#   post.published_at = ""
+#   post.valid?                 # => true
+#
+#   post.published_at = []
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be a string"]
+#
+#   post.published_at = 1
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be a string"]
+#
+#   post.published_at = "2018-09-20 01:02:00 +10:00"
+#   post.valid?                 # => true
+#
+#   post.published_at = "Not Valid"
+#   post.valid?                 # => false
+#   post.errors[:published_at]  # => ["must be valid"]
+class DateTimeStringValidator < ActiveModel::EachValidator
+  NOT_STRING_ERROR = I18n.t("validators.date_time_string_validator.not_string_error")
+  INVALID_FORMAT_ERROR = I18n.t("validators.date_time_string_validator.invalid_format_error")
+
+  def validate_each(record, attribute, value)
+    return if value.nil? || value == ""
+
+    validate_attribute_is_string(record, attribute, value)
+    validate_attribute_is_datetime_string(record, attribute, value)
+  end
+
+  protected
+
+  def validate_attribute_is_string(record, attribute, value)
+    return if value.is_a?(String)
+
+    record.errors.add(attribute, NOT_STRING_ERROR)
+  end
+
+  def validate_attribute_is_datetime_string(record, attribute, value)
+    return unless value.is_a?(String)
+
+    datetime = Time.zone.parse(value)
+    record.errors.add(attribute, INVALID_FORMAT_ERROR) if datetime.blank?
+  end
+end

config/locales/en.yml

@@ -100,6 +100,11 @@ en:
     order_cycle:
       cloned_order_cycle_name: "COPY OF %{order_cycle}"
 
+  validators:
+    date_time_string_validator:
+      not_string_error: "must be a string"
+      invalid_format_error: "must be valid"
+
   enterprise_mailer:
     confirmation_instructions:
       subject: "Please confirm the email address for %{enterprise}"

spec/validators/date_time_string_validator_spec.rb

@@ -0,0 +1,58 @@
+require "spec_helper"
+
+describe DateTimeStringValidator do
+  class TestModel
+    include ActiveModel::Validations
+
+    attr_accessor :timestamp
+
+    validates :timestamp, date_time_string: true
+  end
+
+  describe "internationalization" do
+    it "has translation for NOT_STRING_ERROR" do
+      expect(described_class::NOT_STRING_ERROR).not_to be_blank
+    end
+
+    it "has translation for INVALID_FORMAT_ERROR" do
+      expect(described_class::INVALID_FORMAT_ERROR).not_to be_blank
+    end
+  end
+
+  describe "validation" do
+    let(:instance) { TestModel.new }
+
+    it "does not add error when nil" do
+      instance.timestamp = nil
+      expect(instance).to be_valid
+    end
+
+    it "does not add error when blank string" do
+      instance.timestamp = nil
+      expect(instance).to be_valid
+    end
+
+    it "adds error NOT_STRING_ERROR when blank but neither nil nor a string" do
+      instance.timestamp = []
+      expect(instance).not_to be_valid
+      expect(instance.errors[:timestamp]).to eq([described_class::NOT_STRING_ERROR])
+    end
+
+    it "adds error NOT_STRING_ERROR when not a string" do
+      instance.timestamp = 1
+      expect(instance).not_to be_valid
+      expect(instance.errors[:timestamp]).to eq([described_class::NOT_STRING_ERROR])
+    end
+
+    it "does not add error when value can be parsed" do
+      instance.timestamp = "2018-09-20 01:02:00 +10:00"
+      expect(instance).to be_valid
+    end
+
+    it "adds error INVALID_FORMAT_ERROR when value cannot be parsed" do
+      instance.timestamp = "Not Valid"
+      expect(instance).not_to be_valid
+      expect(instance.errors[:timestamp]).to eq([described_class::INVALID_FORMAT_ERROR])
+    end
+  end
+end