DEV: Rename direct message related models

This is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.

For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.

plugins/chat/spec/models/direct_message_spec.rb

@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe DirectMessage do
+  fab!(:user1) { Fabricate(:user, username: "chatdmfellow1") }
+  fab!(:user2) { Fabricate(:user, username: "chatdmuser") }
+  fab!(:chat_channel) { Fabricate(:direct_message_channel) }
+
+  it_behaves_like "a chatable model" do
+    fab!(:chatable) { Fabricate(:direct_message) }
+    let(:channel_class) { DirectMessageChannel }
+  end
+
+  describe "#chat_channel_title_for_user" do
+    it "returns a nicely formatted name if it's more than one user" do
+      user3 = Fabricate.build(:user, username: "chatdmregent")
+      direct_message = Fabricate(:direct_message, users: [user1, user2, user3])
+
+      expect(direct_message.chat_channel_title_for_user(chat_channel, user1)).to eq(
+        I18n.t(
+          "chat.channel.dm_title.multi_user",
+          users: [user3, user2].map { |u| "@#{u.username}" }.join(", "),
+        ),
+      )
+    end
+
+    it "returns a nicely formatted truncated name if it's more than 5 users" do
+      user3 = Fabricate.build(:user, username: "chatdmregent")
+
+      users = [user1, user2, user3].concat(
+        5.times.map.with_index { |i| Fabricate(:user, username: "chatdmuser#{i}") },
+      )
+      direct_message = Fabricate(:direct_message, users: users)
+
+      expect(direct_message.chat_channel_title_for_user(chat_channel, user1)).to eq(
+        I18n.t(
+          "chat.channel.dm_title.multi_user_truncated",
+          users: users[1..5].sort_by(&:username).map { |u| "@#{u.username}" }.join(", "),
+          leftover: 2,
+        ),
+      )
+    end
+
+    it "returns the other user's username if it's a dm to that user" do
+      direct_message = Fabricate(:direct_message, users: [user1, user2])
+
+      expect(direct_message.chat_channel_title_for_user(chat_channel, user1)).to eq(
+        I18n.t("chat.channel.dm_title.single_user", user: "@#{user2.username}"),
+      )
+    end
+
+    it "returns the current user's username if it's a dm to self" do
+      direct_message = Fabricate(:direct_message, users: [user1])
+
+      expect(direct_message.chat_channel_title_for_user(chat_channel, user1)).to eq(
+        I18n.t("chat.channel.dm_title.single_user", user: "@#{user1.username}"),
+      )
+    end
+
+    context "when user is deleted" do
+      it "returns a placeholder username" do
+        direct_message = Fabricate(:direct_message, users: [user1, user2])
+        user2.destroy!
+        direct_message.reload
+
+        expect(direct_message.chat_channel_title_for_user(chat_channel, user1)).to eq(
+          "@#{I18n.t("chat.deleted_chat_username")}",
+        )
+      end
+    end
+  end
+end