diff --git a/aggify/aggify.py b/aggify/aggify.py
index d403623..5b16734 100644
--- a/aggify/aggify.py
+++ b/aggify/aggify.py
@@ -84,13 +84,13 @@ def project(self, **kwargs: QueryParams) -> "Aggify":
         Returns:
             Aggify: Returns an instance of the Aggify class for potential method chaining.
         """
-
-        if all([i in kwargs.values() for i in [0, 1]]):
+        filtered_kwargs = dict(kwargs)
+        filtered_kwargs.pop("id", None)
+        if all([i in filtered_kwargs.values() for i in [0, 1]]):
             raise InvalidProjection()
 
         # Extract fields to keep and check if _id should be deleted
         to_keep_values = {"id"}
-        delete_id = kwargs.get("id") is not None
         projection = {}
 
         # Add missing fields to the base model
@@ -109,13 +109,10 @@ def project(self, **kwargs: QueryParams) -> "Aggify":
         # Remove fields from the base model, except the ones in to_keep_values and possibly _id
         if to_keep_values != {"id"}:
             keys_for_deletion = self.base_model._fields.keys() - to_keep_values  # noqa
-            if delete_id:
-                keys_for_deletion.add("id")
             for key in keys_for_deletion:
                 del self.base_model._fields[key]  # noqa
         # Append the projection stage to the pipelines
         self.pipelines.append({"$project": projection})
-
         # Return the instance for method chaining
         return self
 
diff --git a/tests/test_aggify.py b/tests/test_aggify.py
index f931deb..cc71fcf 100644
--- a/tests/test_aggify.py
+++ b/tests/test_aggify.py
@@ -51,12 +51,14 @@ def test_filtering_and_projection(self):
         aggify.filter(age__gte=30).project(name=1, age=1)
         assert len(aggify.pipelines) == 2
         assert aggify.pipelines[1]["$project"] == {"name": 1, "age": 1}
+        assert list(aggify.base_model._fields.keys()) == ["name", "age", "id"]
 
     def test_filtering_and_projection_with_deleting_id(self):
         aggify = Aggify(BaseModel)
-        aggify.filter(age__gte=30).project(name=1, age=1, id=1)
+        aggify.filter(age__gte=30).project(name=1, age=1, id=0)
         assert len(aggify.pipelines) == 2
-        assert aggify.pipelines[1]["$project"] == {"_id": 1, "name": 1, "age": 1}
+        assert aggify.pipelines[1]["$project"] == {"_id": 0, "name": 1, "age": 1}
+        assert list(aggify.base_model._fields.keys()) == ["name", "age"]
 
     def test_filtering_and_ordering(self):
         aggify = Aggify(BaseModel)
@@ -636,3 +638,9 @@ def test_project_use_inclusion_and_exclusion_together(self):
         with pytest.raises(InvalidProjection):
             # noinspection PyUnusedLocal
             var = aggify.project(name=0, age=1)
+
+    def test_project_add_new_field(self):
+        aggify = Aggify(BaseModel)
+        thing = list(aggify.project(test="test", id=0))
+        assert thing[0]["$project"] == {"test": "test", "_id": 0}
+        assert list(aggify.base_model._fields.keys()) == ["test"]