added ldap validator and spec

app/validators/ldap_filter_validator.rb

+# LdapFilteralidator
+#
+# Custom validator for LDAP filters
+#
+# Example:
+#
+#   class LdapGroupLink < ActiveRecord::Base
+#     validates :filter, ldap_filter: true
+#   end
+#
+class LdapFilterValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    Net::LDAP::Filter::FilterParser.parse(value)
+  rescue Net::LDAP::FilterSyntaxInvalidError
+    record.errors.add(attribute, 'must be a valid filter')
+  end
+end

spec/validators/ldap_filter_validator_spec.rb

+require 'spec_helper'
+
+describe LdapFilterValidator do
+  let(:validator) { described_class.new(attributes: [:filter]) }
+
+  describe '#validates_each' do
+    it 'adds a message when the filter is not valid' do
+      link = build(:ldap_group_link, cn: nil)
+
+      validator.validate_each(link, :filter, 'wrong filter')
+
+      expect(link.errors[:filter]).to match_array(['must be a valid filter'])
+    end
+
+    it 'has no errors when is valid' do
+      link = build(:ldap_group_link, cn: nil)
+
+      validator.validate_each(link, :filter, '(cn=Babs Jensen)')
+
+      expect(link.errors[:filter]).to eq([])
+    end
+  end
+end