DEV: Extract leave logic to the Chat::Channel model

Extracted from https://github.com/discourse/discourse/pull/29129.

plugins/chat/app/models/chat/channel.rb

@@ -141,6 +141,7 @@ module Chat
     def remove(user)
       Chat::ChannelMembershipManager.new(self).unfollow(user)
     end
+    alias leave remove
 
     def url
       "#{Discourse.base_url}/chat/c/#{self.slug || "-"}/#{self.id}"

plugins/chat/app/models/chat/direct_message_channel.rb

@@ -3,6 +3,9 @@
 module Chat
   class DirectMessageChannel < Channel
     alias_method :direct_message, :chatable
+
+    delegate :group?, to: :direct_message, prefix: true, allow_nil: true
+
     before_validation(on: :create) { self.threading_enabled = true }
 
     def direct_message_channel?
@@ -24,5 +27,13 @@ module Chat
     def generate_auto_slug
       self.slug.blank?
     end
+
+    def leave(user)
+      return super unless direct_message_group?
+      transaction do
+        membership_for(user)&.destroy!
+        direct_message.users.delete(user)
+      end
+    end
   end
 end

plugins/chat/app/services/chat/leave_channel.rb

@@ -33,14 +33,7 @@ module Chat
     end
 
     def leave(channel:, guardian:)
-      ActiveRecord::Base.transaction do
-        if channel.direct_message_channel? && channel.chatable&.group
-          channel.membership_for(guardian.user)&.destroy!
-          channel.chatable.direct_message_users.where(user_id: guardian.user.id).destroy_all
-        else
-          channel.remove(guardian.user)
-        end
-      end
+      channel.leave(guardian.user)
     end
 
     def recompute_users_count(channel:)

plugins/chat/spec/models/chat/category_channel_spec.rb

@@ -88,6 +88,16 @@ RSpec.describe Chat::CategoryChannel do
     end
   end
 
+  describe "#leave" do
+    let(:original_method) { channel.method(:remove) }
+    let(:aliased_method) { channel.method(:leave) }
+
+    it "is an alias to '#remove'" do
+      expect(original_method.original_name).to eq(aliased_method.original_name)
+      expect(original_method.source_location).to eq(aliased_method.source_location)
+    end
+  end
+
   describe "slug generation" do
     subject(:channel) { Fabricate(:category_channel) }
 

plugins/chat/spec/models/chat/direct_message_channel_spec.rb

@@ -6,6 +6,7 @@ RSpec.describe Chat::DirectMessageChannel do
   it_behaves_like "a chat channel model"
 
   it { is_expected.to delegate_method(:allowed_user_ids).to(:direct_message).as(:user_ids) }
+  it { is_expected.to delegate_method(:group?).to(:direct_message).with_prefix.allow_nil }
   it { is_expected.to validate_length_of(:description).is_at_most(500) }
   it { is_expected.to validate_length_of(:chatable_type).is_at_most(100) }
   it { is_expected.to validate_length_of(:type).is_at_most(100) }
@@ -70,6 +71,36 @@ RSpec.describe Chat::DirectMessageChannel do
     end
   end
 
+  describe "#leave" do
+    subject(:leave) { channel.leave(user) }
+
+    let(:channel) { Fabricate(:direct_message_channel, group:) }
+    let(:user) { channel.chatable.users.first }
+    let(:membership) { channel.membership_for(user) }
+
+    context "when DM is not a group" do
+      let(:group) { false }
+
+      it "unfollows the channel for the provided user" do
+        expect { leave }.to change { membership.reload.following? }.to(false)
+      end
+    end
+
+    context "when DM is a group" do
+      let(:group) { true }
+
+      it "destroys the provided user’s membership" do
+        expect { leave }.to change { channel.user_chat_channel_memberships.where(user:).count }.by(
+          -1,
+        )
+      end
+
+      it "removes the provided user from the DM" do
+        expect { leave }.to change { channel.chatable.users.where(id: user).count }.by(-1)
+      end
+    end
+  end
+
   describe "slug generation" do
     subject(:channel) { Fabricate(:direct_message_channel) }