Extract methods to get recipients and valid segments

This way we don't have to use the `send` method in other places, like
the AdminNotification class, and we can change the internal
implementation at any point.

app/models/admin_notification.rb

@@ -13,11 +13,11 @@ class AdminNotification < ApplicationRecord
   before_validation :complete_link_url
 
   def list_of_recipients
-    UserSegments.send(segment_recipient) if valid_segment_recipient?
+    UserSegments.recipients(segment_recipient) if valid_segment_recipient?
   end
 
   def valid_segment_recipient?
-    segment_recipient && UserSegments.respond_to?(segment_recipient)
+    UserSegments.valid_segment?(segment_recipient)
   end
 
   def draft?

app/models/newsletter.rb

@@ -15,7 +15,7 @@ class Newsletter < ApplicationRecord
   end
 
   def valid_segment_recipient?
-    segment_recipient && UserSegments.respond_to?(segment_recipient)
+    UserSegments.valid_segment?(segment_recipient)
   end
 
   def draft?

lib/user_segments.rb

@@ -57,8 +57,16 @@ class UserSegments
     )
   end
 
+  def self.valid_segment?(segment)
+    segment && respond_to?(segment)
+  end
+
+  def self.recipients(segment)
+    send(segment)
+  end
+
   def self.user_segment_emails(segment)
-    UserSegments.send(segment).newsletter.order(:created_at).pluck(:email).compact
+    recipients(segment).newsletter.order(:created_at).pluck(:email).compact
   end
 
   private

spec/lib/user_segments_spec.rb

@@ -21,6 +21,28 @@ describe UserSegments do
     end
   end
 
+  describe ".valid_segment?" do
+    it "returns true when the segment exists" do
+      expect(UserSegments.valid_segment?("all_proposal_authors")).to be true
+      expect(UserSegments.valid_segment?("investment_authors")).to be true
+      expect(UserSegments.valid_segment?("feasible_and_undecided_investment_authors")).to be true
+    end
+
+    it "accepts symbols as parameters" do
+      expect(UserSegments.valid_segment?(:selected_investment_authors)).to be true
+      expect(UserSegments.valid_segment?(:winner_investment_authors)).to be true
+      expect(UserSegments.valid_segment?(:not_supported_on_current_budget)).to be true
+    end
+
+    it "is falsey when the segment doesn't exist" do
+      expect(UserSegments.valid_segment?("imaginary_segment")).to be_falsey
+    end
+
+    it "is falsey when nil is passed" do
+      expect(UserSegments.valid_segment?(nil)).to be_falsey
+    end
+  end
+
   describe ".all_users" do
     it "returns all active users enabled" do
       active_user = create(:user)