document the sandwich pattern (#252)

Changes:

- document the sandwich pattern

docs/tips-and-tricks.md

@@ -62,13 +62,37 @@ the `lru_cache` technique for our `db_connection`.
 This will make sure that from now on you will always use the same connection and registry within
 your appliction by importing the `get_db_connection()` anywhere is needed.
 
-Why don't we use `edgy.monkay.instance.registry` instead? It is a chicken-egg problem:
+Note, you cannot do that if `get_db_connection()` is in the same file like the application entrypoint.
+Here you can use a [`edgy.monkay.instance`](#excurse-the-edgymonkayinstance-sandwich) sandwich instead.
 
-It is not set before the preloads are executed. You are running into circular import issues.
+You can also read further the [Practical Example](#practical-example).
 
-There is also a second advantage of using the lru cache: you can have multiple registries.
+## Excurse: The `edgy.monkay.instance` sandwich
 
-## Pratical example
+If you want to short down the code and concentrate in e.g. `main.py` you can also use manual post loads and do the initialization in
+`get_application` this way:
+
+1. Creating registry.
+2. Assigning the Instance to edgy.instance via set_instance() but without app and skip extensions.
+3. Post loading models.
+4. Creating the main app.
+5. Assigning the Instance to edgy.instance via set_instance() but with app.
+
+this looks like:
+
+```` python title="main.py"
+{!> ../docs_src/tips/sandwich_main.py !}
+````
+
+```` python title="myproject/models.py"
+{!> ../docs_src/tips/sandwich_models.py !}
+````
+
+The sandwich way has the disadvantage of having just one registry, while with the lru_cache way you can have many
+registries in parallel and mix them.
+
+
+## Practical example
 
 Let us now assemble everything and generate an application that will have:
 

docs_src/tips/sandwich_main.py

@@ -0,0 +1,25 @@
+from importlib import import_module
+
+import edgy
+from esmerald import Esmerald
+
+
+def setup():
+    # do preparations
+    ...
+
+
+def get_application():
+    setup()
+    registry = edgy.Registry(url=...)
+    # extensions shouldn't be applied yet
+    edgy.monkay.set_instance(edgy.Instance(registry=registry), apply_extensions=False)
+    # post loads
+    import_module("myproject.models")
+    app = Esmerald()
+    # now apply the extensions
+    edgy.monkay.set_instance(edgy.Instance(registry=registry, app=app))
+    return app
+
+
+application = get_application()

docs_src/tips/sandwich_models.py

@@ -0,0 +1,42 @@
+from datetime import datetime
+from enum import Enum
+
+import edgy
+
+registry = edgy.monkay.instance.registry
+
+
+class ProfileChoice(Enum):
+    ADMIN = "ADMIN"
+    USER = "USER"
+
+
+class BaseModel(edgy.Model):
+    class Meta:
+        abstract = True
+        registry = registry
+
+
+class User(BaseModel):
+    """
+    Base model for a user
+    """
+
+    first_name: str = edgy.CharField(max_length=150)
+    last_name: str = edgy.CharField(max_length=150)
+    username: str = edgy.CharField(max_length=150, unique=True)
+    email: str = edgy.EmailField(max_length=120, unique=True)
+    password: str = edgy.CharField(max_length=128)
+    last_login: datetime = edgy.DateTimeField(null=True)
+    is_active: bool = edgy.BooleanField(default=True)
+    is_staff: bool = edgy.BooleanField(default=False)
+    is_superuser: bool = edgy.BooleanField(default=False)
+
+
+class Profile(BaseModel):
+    """
+    A profile for a given user.
+    """
+
+    user: User = edgy.OneToOneField(User, on_delete=edgy.CASCADE)
+    profile_type: ProfileChoice = edgy.ChoiceField(ProfileChoice, default=ProfileChoice.USER)