Create helper to set blank attributes to nil before validation

Uses a before_validation callback to set configured
attributes to nil if they have a truthy response to #blank?,
which helps prevent empty strings ending up in the database.

app/models/concerns/nullify_if_blank.rb

+# frozen_string_literal: true
+
+# Helper that sets attributes to nil prior to validation if they
+# are blank (are false, empty or contain only whitespace), to avoid
+# unnecessarily persisting empty strings.
+#
+# Model usage:
+#
+#   class User < ApplicationRecord
+#     include NullifyIfBlank
+#
+#     nullify_if_blank :name, :email
+#   end
+#
+#
+# Test usage:
+#
+#   RSpec.describe User do
+#     it { is_expected.to nullify_if_blank(:name) }
+#     it { is_expected.to nullify_if_blank(:email) }
+#   end
+#
+module NullifyIfBlank
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def nullify_if_blank(*attributes)
+      self.attributes_to_nullify += attributes
+    end
+  end
+
+  included do
+    class_attribute :attributes_to_nullify,
+      instance_accessor: false,
+      instance_predicate: false,
+      default: Set.new
+
+    before_validation :nullify_blank_attributes
+  end
+
+  private
+
+  def nullify_blank_attributes
+    self.class.attributes_to_nullify.each do |attribute|
+      assign_attributes(attribute => nil) if read_attribute(attribute).blank?
+    end
+  end
+end

spec/models/concerns/nullify_if_blank_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe NullifyIfBlank do
+  let_it_be(:model) do
+    Class.new(ApplicationRecord) do
+      include NullifyIfBlank
+
+      nullify_if_blank :name
+
+      self.table_name = 'users'
+    end
+  end
+
+  context 'attribute exists' do
+    let(:instance) { model.new(name: name) }
+
+    subject { instance.name }
+
+    before do
+      instance.validate
+    end
+
+    context 'attribute is blank' do
+      let(:name) { '' }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'attribute is nil' do
+      let(:name) { nil }
+
+      it { is_expected.to be_nil}
+    end
+
+    context 'attribute is not blank' do
+      let(:name) { 'name' }
+
+      it { is_expected.to eq('name') }
+    end
+  end
+
+  context 'attribute does not exist' do
+    before do
+      model.table_name = 'issues'
+    end
+
+    it { expect { model.new.valid? }.to raise_error(ActiveModel::UnknownAttributeError) }
+  end
+end

spec/support/matchers/nullify_if_blank_matcher.rb

+# frozen_string_literal: true
+
+RSpec::Matchers.define :nullify_if_blank do |attribute|
+  match do |record|
+    expect(record.class.attributes_to_nullify).to include(attribute)
+  end
+
+  failure_message do |record|
+    "expected nullify_if_blank configuration on #{record.class} to include #{attribute}"
+  end
+end