Fix #5001: Media assets not searchable through upload records.

Fix this:

  https://danbooru.donmai.us/uploads.json?search[media_assets][md5]=b83daa7f1ae7e4127b1befd32f71ba10

failing with an ActiveRecord::StatementInvalid error.

The bug was that for a `has_many through: ...` association, like
`has_many :media_assets, through: :upload_media_assets`, we weren't
joining on the associated table properly so we ended up generating
invalid SQL.

test/unit/concerns/searchable.rb

@@ -252,5 +252,17 @@ class SearchableTest < ActiveSupport::TestCase
         assert_search_equals(@p1, has_comments: true, comments: { id: @p1.comments.first.id })
       end
     end
+
+    context "for a `has_many through: ...` association" do
+      subject { Upload }
+
+      should "work" do
+        @media_asset = create(:media_asset)
+        @upload1 = create(:upload, media_assets: [@media_asset])
+        @upload2 = create(:upload, media_assets: [@media_asset])
+
+        assert_search_equals([@upload2, @upload1], media_asset: { md5: @media_asset.md5 })
+      end
+    end
   end
 end