Can preload has_one associations with existing records

spec/preloading/preloading_has_one_spec.cr

@@ -78,4 +78,55 @@ describe "Preloading has_one associations" do
 
     SignInCredential::BaseQuery.times_called.should eq 0
   end
+
+  context "with existing record" do
+    it "works" do
+      with_lazy_load(enabled: false) do
+        admin = AdminBox.create
+        sign_in_credential = SignInCredentialBox.create &.user_id(admin.id)
+
+        admin = Admin::BaseQuery.preload_sign_in_credential(admin)
+
+        admin.sign_in_credential.should eq sign_in_credential
+      end
+    end
+
+    it "works with multiple" do
+      with_lazy_load(enabled: false) do
+        admin = AdminBox.create
+        sign_in_credential = SignInCredentialBox.create &.user_id(admin.id)
+        admin2 = AdminBox.create
+        sign_in_credential2 = SignInCredentialBox.create &.user_id(admin2.id)
+
+        admins = Admin::BaseQuery.preload_sign_in_credential([admin, admin2])
+
+        admins[0].sign_in_credential.should eq(sign_in_credential)
+        admins[1].sign_in_credential.should eq(sign_in_credential2)
+      end
+    end
+
+    it "works with custom query" do
+      with_lazy_load(enabled: false) do
+        user = UserBox.create
+        sign_in_credential = SignInCredentialBox.create &.user_id(user.id)
+
+        user = UserQuery.preload_sign_in_credential(user, SignInCredential::BaseQuery.new.id.not.eq(sign_in_credential.id))
+
+        user.sign_in_credential.should be_nil
+      end
+    end
+
+    it "does not modify original record" do
+      with_lazy_load(enabled: false) do
+        admin = AdminBox.create
+        SignInCredentialBox.create &.user_id(admin.id)
+
+        Admin::BaseQuery.preload_sign_in_credential(admin)
+
+        expect_raises Avram::LazyLoadError do
+          admin.sign_in_credential
+        end
+      end
+    end
+  end
 end

src/avram/associations/has_one.cr

@@ -45,6 +45,21 @@ module Avram::Associations::HasOne
 
   private macro define_has_one_base_query(assoc_name, model, foreign_key)
     class BaseQuery
+      def self.preload_{{ assoc_name }}(record, preload_query = {{ model }}::BaseQuery.new)
+        preload_{{ assoc_name }}(records: [record], preload_query: preload_query).first
+      end
+
+      def self.preload_{{ assoc_name }}(records : Enumerable, preload_query = {{ model }}::BaseQuery.new)
+        ids = records.map(&.id)
+        empty_results = {} of {{ model }}::PrimaryKeyType => Array({{ model }})
+        {{ assoc_name }} = ids.empty? ? empty_results : preload_query.{{ foreign_key }}.in(ids).results.group_by(&.{{ foreign_key }})
+        records.map(&.dup)
+          .map do |record|
+            assoc = {{ assoc_name }}[record.id]?.try(&.first?)
+            record.tap(&.__set_preloaded_{{ assoc_name }}(assoc))
+          end
+      end
+
       def preload_{{ assoc_name }}
         preload_{{ assoc_name }}({{ model }}::BaseQuery.new)
       end
@@ -53,7 +68,7 @@ module Avram::Associations::HasOne
         add_preload do |records|
           ids = records.map(&.id)
           empty_results = {} of {{ model }}::PrimaryKeyType => Array({{ model }})
-          {{ assoc_name }} = ids.empty? ? empty_results : preload_query.dup.{{ foreign_key }}.in(ids).results.group_by(&.{{ foreign_key }})
+          {{ assoc_name }} = ids.empty? ? empty_results : preload_query.{{ foreign_key }}.in(ids).results.group_by(&.{{ foreign_key }})
           records.each do |record|
             record.__set_preloaded_{{ assoc_name }} {{ assoc_name }}[record.id]?.try(&.first?)
           end