diff --git a/cascade/base/__init__.py b/cascade/base/__init__.py
index 692a5d92..b2404e65 100644
--- a/cascade/base/__init__.py
+++ b/cascade/base/__init__.py
@@ -47,5 +47,5 @@ def raise_not_implemented(class_name: str, name: str) -> NoReturn:
 
 from .history_logger import HistoryLogger
 from .meta_handler import CustomEncoder as JSONEncoder
-from .meta_handler import MetaHandler, supported_meta_formats
+from .meta_handler import MetaHandler, default_meta_format, supported_meta_formats
 from .traceable import Traceable, TraceableOnDisk
diff --git a/cascade/base/meta_handler.py b/cascade/base/meta_handler.py
index a835da5f..a2c653d2 100644
--- a/cascade/base/meta_handler.py
+++ b/cascade/base/meta_handler.py
@@ -26,6 +26,7 @@
 
 from . import MetaFromFile
 
+default_meta_format = ".json"
 supported_meta_formats = (".json", ".yml", ".yaml")
 
 
diff --git a/cascade/base/traceable.py b/cascade/base/traceable.py
index 702bbeaa..348069f7 100644
--- a/cascade/base/traceable.py
+++ b/cascade/base/traceable.py
@@ -24,7 +24,7 @@
 import pendulum
 from datetime import datetime
 
-from . import PipeMeta, MetaFromFile, supported_meta_formats
+from . import PipeMeta, MetaFromFile, default_meta_format, supported_meta_formats
 
 
 @dataclass
@@ -265,16 +265,42 @@ class TraceableOnDisk(Traceable):
     def __init__(
         self,
         root: str,
-        meta_fmt: Literal[".json", ".yml", ".yaml"],
+        meta_fmt: Literal[".json", ".yml", ".yaml", None],
         *args: Any,
         meta_prefix: Union[Dict[Any, Any], str, None] = None,
         **kwargs: Any,
     ) -> None:
         super().__init__(*args, meta_prefix=meta_prefix, **kwargs)
         self._root = root
-        if meta_fmt not in supported_meta_formats:
-            raise ValueError(f"Only {supported_meta_formats} are supported formats")
-        self._meta_fmt = meta_fmt
+        
+        ext = self._determine_meta_fmt()
+
+        if ext is None and meta_fmt is None:
+            meta_fmt = default_meta_format
+        elif not ext:
+            # Here we write meta first time and
+            # don't know the real ext from file
+            if meta_fmt not in supported_meta_formats:
+                raise ValueError(f"Only {supported_meta_formats} are supported formats")
+            self._meta_fmt = meta_fmt
+        else:
+            # Here we know the real extension and will
+            # strictly use it regardless of what was passed
+            self._meta_fmt = ext
+            if meta_fmt != ext:
+                warnings.warn(
+                    f"Trying to set {meta_fmt} to the object that already has {ext} on path {self._root}"
+                )
+
+    def _determine_meta_fmt(self) -> Union[str, None]:
+        meta_paths = glob.glob(os.path.join(self._root, "meta.*"))
+        if len(meta_paths) == 1:
+            _, ext = os.path.splitext(meta_paths[0])
+            return ext
+        else:
+            warnings.warn(
+                f"Multiple meta files found in {self._root}"
+            )
 
     def _create_meta(self) -> None:
         meta_path = sorted(glob.glob(os.path.join(self._root, "meta.*")))
diff --git a/cascade/models/model_line.py b/cascade/models/model_line.py
index 79e4bb51..4476081d 100644
--- a/cascade/models/model_line.py
+++ b/cascade/models/model_line.py
@@ -36,7 +36,7 @@ def __init__(
         self,
         folder: str,
         model_cls: Type = Model,
-        meta_fmt: Literal[".json", ".yml", ".yaml"] = ".json",
+        meta_fmt: Literal[".json", ".yml", ".yaml", None] = None,
         **kwargs: Any,
     ) -> None:
         """
@@ -49,7 +49,7 @@ def __init__(
             If folder does not exist, creates it
         model_cls: type, optional
             A class of models in line. ModelLine uses this class to reconstruct a model
-        meta_fmt: Literal[".json", ".yml", ".yaml"], optional
+        meta_fmt: Literal[".json", ".yml", ".yaml", None], optional
             Format in which to store meta data.
         See also
         --------
diff --git a/cascade/models/model_repo.py b/cascade/models/model_repo.py
index 36ec0843..4636a20a 100644
--- a/cascade/models/model_repo.py
+++ b/cascade/models/model_repo.py
@@ -44,10 +44,6 @@ def __init__(
         super().__init__(*args, meta_prefix=meta_prefix, **kwargs)
         self._lines = dict()
 
-    def reload(self) -> None:
-        for line in self._lines:
-            self._lines[line].reload()
-
     def __getitem__(self, key: str) -> ModelLine:
         raise NotImplementedError()
 
@@ -75,6 +71,9 @@ def get_line_names(self) -> List[str]:
         """
         return list(self._lines.keys())
 
+    def reload(self) -> None:
+        pass
+
 
 class SingleLineRepo(Repo):
     def __init__(
@@ -86,10 +85,15 @@ def __init__(
     ) -> None:
         self._root = line.get_root()
         super().__init__(*args, meta_prefix=meta_prefix, **kwargs)
-        self._lines = {os.path.split(self._root)[-1]: line}
+        self._lines = {os.path.split(self._root)[-1]: {}}
+        self._line = line
 
     def __getitem__(self, key: str) -> ModelLine:
-        return self._lines[key]
+        if key in self._lines:
+            return self._line
+        else:
+            raise KeyError(
+                f"The only line is {list(self._lines.keys())[0]}, {key} does not exist")
 
     def __repr__(self) -> str:
         return f"SingleLine in {self._root}"
@@ -162,26 +166,23 @@ def __init__(
             shutil.rmtree(self._root)
 
         os.makedirs(self._root, exist_ok=True)
-        self._load_lines()
+        self._lines = {
+            name: {}
+            for name in sorted(os.listdir(self._root))
+            if os.path.isdir(os.path.join(self._root, name))
+        }
 
         if lines is not None:
             for line in lines:
-                self.add_line(**line)
+                name = line["name"]
+                del line["name"]
 
-        self._create_meta()
+                self._lines[name] = {
+                    "args": [],
+                    "kwargs": line
+                }
 
-    def _load_lines(self) -> None:
-        self._lines = {
-            name: ModelLine(
-                os.path.join(self._root, name),
-                model_cls=self._model_cls
-                if isinstance(self._model_cls, type)
-                else self._model_cls[name],
-                meta_fmt=self._meta_fmt,
-            )
-            for name in sorted(os.listdir(self._root))
-            if os.path.isdir(os.path.join(self._root, name))
-        }
+        self._create_meta()
 
     def add_line(
         self,
@@ -223,10 +224,17 @@ def add_line(
         folder = os.path.join(self._root, name)
         if meta_fmt is None:
             meta_fmt = self._meta_fmt
-        line = ModelLine(folder, *args, meta_fmt=meta_fmt, **kwargs)
-        self._lines[name] = line
 
+        self._lines[name] = {
+            "args": args,
+            "kwargs": {
+                "meta_fmt": meta_fmt,
+                **kwargs
+            }
+        }
         self._update_meta()
+
+        line = ModelLine(folder, *args, meta_fmt=meta_fmt, **kwargs)
         return line
 
     def __getitem__(self, key: Union[str, int]) -> ModelLine:
@@ -236,13 +244,20 @@ def __getitem__(self, key: Union[str, int]) -> ModelLine:
         line: ModelLine
            existing line of the name passed in `key`
         """
-        if isinstance(key, str):
-            return self._lines[key]
-        elif isinstance(key, int):
-            return self._lines[list(self._lines.keys())[key]]
-        else:
+        if isinstance(key, int):
+            key = list(self._lines.keys())[key]
+        elif not isinstance(key, str):
             raise TypeError(f"{type(key)} is not supported as key")
 
+        if key in self._lines:
+            return ModelLine(
+                os.path.join(self._root, key),
+                *self._lines[key]["args"],
+                **self._lines[key]["kwargs"],
+            )
+        else:
+            raise KeyError(f"Line {key} does not exist in {self}")
+
     def __repr__(self) -> str:
         return f"ModelRepo in {self._root} of {len(self)} lines"
 
@@ -277,8 +292,9 @@ def load_model_meta(self, model: str) -> MetaFromFile:
             Raises if failed to find the model with slug specified
         """
 
-        for line in self._lines.values():
+        for name in self._lines:
             try:
+                line = ModelLine(os.path.join(self._root, name), *self._lines[name]["args"], **self._lines[name]["kwargs"])
                 meta = line.load_model_meta(model)
             except FileNotFoundError:
                 continue
@@ -294,13 +310,10 @@ def _update_lines(self) -> None:
                 os.path.isdir(os.path.join(self._root, name))
                 and name not in self._lines
             ):
-                self._lines[name] = ModelLine(
-                    os.path.join(self._root, name),
-                    model_cls=self._model_cls
-                    if isinstance(self._model_cls, type)
-                    else self._model_cls[name],
-                    meta_fmt=self._meta_fmt,
-                )
+                self._lines[name] = {
+                    "args": [],
+                    "kwargs": dict()
+                }
 
 
 class ModelRepoConcatenator(Repo):
diff --git a/cascade/tests/test_model_repo.py b/cascade/tests/test_model_repo.py
index a2920cac..14e8371b 100644
--- a/cascade/tests/test_model_repo.py
+++ b/cascade/tests/test_model_repo.py
@@ -254,7 +254,7 @@ def test_failed_line_meta(tmp_path, ext):
         f.write("\t{{{: 'sorry, i am broken'")
 
     repo = ModelRepo(
-        repo_path, lines=[dict(name="0", model_cls=DummyModel, meta_fmt=ext)]
+        repo_path, lines=[dict(name="0", model_cls=DummyModel)], meta_fmt=ext
     )
     model = repo["0"][0]
 
@@ -299,8 +299,8 @@ def test_integer_indices(tmp_path, ext):
     first_line = repo.add_line("a")
     last_line = repo.add_line("b")
 
-    assert first_line == repo[0]
-    assert last_line == repo[-1]
+    assert first_line.get_root() == repo[0].get_root()
+    assert last_line.get_root() == repo[-1].get_root()
 
 
 @pytest.mark.parametrize("ext", [".json", ".yml", ".yaml"])