DEV: Allow specifying a condition when preloading topics for topic list (#32101)

Currently when using `register_topic_preloader_associations`, we are
not able to specify a condition that is evaluated at runtime.

This commit allows specifying a condition and also keeps backward
compatibility.

```
   register_topic_preloader_associations(:linked_topic) { true }
   register_topic_preloader_associations({
     first_post: [uploads]
   })
```

app/models/topic_list.rb

@@ -138,7 +138,15 @@ class TopicList
       { category: :parent_category },
     ]
 
-    topic_preloader_associations.concat(DiscoursePluginRegistry.topic_preloader_associations.to_a)
+    DiscoursePluginRegistry.topic_preloader_associations.each do |a|
+      fields = a[:fields]
+      condition = a[:condition]
+      if condition.present?
+        topic_preloader_associations << fields if condition.present? && condition.call
+      else
+        topic_preloader_associations << fields
+      end
+    end
 
     ActiveRecord::Associations::Preloader.new(
       records: @topics,

lib/plugin/instance.rb

@@ -1347,6 +1347,24 @@ class Plugin::Instance
     end
   end
 
+  # This method allows plugins to preload topic associations when loading topics
+  # that make use of topic_list.
+  #
+  # @param fields [Symbol, Array<Symbol>, Hash] The topic associations to preload.
+  #
+  # @example
+  #   register_topic_preloader_associations(:first_post)
+  #   register_topic_preloader_associations([:first_post, :topic_embeds])
+  #   register_topic_preloader_associations({ first_post: :uploads })
+  #   register_topic_preloader_associations({ first_post: :uploads }) do
+  #     SiteSetting.some_setting_enabled?
+  #   end
+  #
+  # @return [void]
+  def register_topic_preloader_associations(fields, &condition)
+    DiscoursePluginRegistry.register_topic_preloader_association({ fields:, condition: }, self)
+  end
+
   protected
 
   def self.js_path
@@ -1441,10 +1459,6 @@ class Plugin::Instance
     reloadable_patch { NewPostManager.add_plugin_payload_attribute(attribute_name) }
   end
 
-  def register_topic_preloader_associations(fields)
-    DiscoursePluginRegistry.register_topic_preloader_association(fields, self)
-  end
-
   ##
   # Allows plugins to preload topic associations when loading categories with topics.
   #

spec/lib/topic_query_spec.rb

@@ -1305,17 +1305,22 @@ RSpec.describe TopicQuery do
 
     context "when preloading associations" do
       it "preloads associations" do
-        DiscoursePluginRegistry.register_topic_preloader_association(
-          :first_post,
-          Plugin::Instance.new,
-        )
+        plugin = Plugin::Instance.new
+        plugin.register_topic_preloader_associations(:topic_embed)
+        plugin.register_topic_preloader_associations({ first_post: [:uploads] })
+        plugin.register_topic_preloader_associations(:user_warning) { true }
+        plugin.register_topic_preloader_associations(:linked_topic) { false }
 
         topic = Fabricate(:topic)
         Fabricate(:post, topic: topic)
 
         new_topic = topic_query.list_new.topics.first
         expect(new_topic.association(:image_upload).loaded?).to eq(true) # Preloaded by default
-        expect(new_topic.association(:first_post).loaded?).to eq(true) # Testing a user-defined preloaded association
+        expect(new_topic.association(:topic_embed).loaded?).to eq(true) # Testing a user-defined preloaded association
+        expect(new_topic.association(:first_post).loaded?).to eq(true) # Nested preloaded association
+        expect(new_topic.first_post.association(:uploads).loaded?).to eq(true) # Nested preloaded association
+        expect(new_topic.association(:user_warning).loaded?).to eq(true) # Conditionally loaded
+        expect(new_topic.association(:linked_topic).loaded?).to eq(false) # Failed condition
         expect(new_topic.association(:user).loaded?).to eq(false) # Testing the negative
 
         DiscoursePluginRegistry.reset_register!(:topic_preloader_associations)