Migration Guide: Add Model initialization requirement and automatic Agent.unique_id assignment

docs/migration_guide.md

@@ -29,10 +29,49 @@ The `mesa.flat` namespace is removed. Use the full namespace for your imports.
 - Ref: [PR #2091](https://github.com/projectmesa/mesa/pull/2091)
 
 
-### Automatic assignment of `unique_id` to Agents
-<!-- TODO -->
+### Mandatory Model initialization with `super().__init__()`
+In Mesa 3.0, it is now mandatory to call `super().__init__()` when initializing your model class. This ensures that all necessary Mesa model variables are correctly set up and agents are properly added to the model.
+
+Make sure all your model classes explicitly call `super().__init__()` in their `__init__` method:
+
+```python
+class MyModel(mesa.Model):
+    def __init__(self, *args, **kwargs):
+        super().__init__()  # This is now required!
+        # Your model initialization code here
+```
+
+This change ensures that all Mesa models are properly initialized, which is crucial for:
+- Correctly adding agents to the model
+- Setting up other essential Mesa model variables
+- Maintaining consistency across all models
+
+If you forget to call `super().__init__()`, you'll now see this error:
 
-- Ref: [PR #2226](https://github.com/projectmesa/mesa/pull/2226)
+```
+RuntimeError: The Mesa Model class was not initialized. You must explicitly initialize the Model by calling super().__init__() on initialization.
+```
+
+- Ref: [PR #2218](https://github.com/projectmesa/mesa/pull/2218), [PR #1928](https://github.com/projectmesa/mesa/pull/1928), Mesa-examples [PR #83](https://github.com/projectmesa/mesa-examples/pull/83)
+
+
+### Automatic assignment of `unique_id` to Agents
+In Mesa 3.0, `unique_id` for agents is now automatically assigned, simplifying agent creation and ensuring unique IDs across all agents in a model.
+
+1. Remove `unique_id` from agent initialization:
+   ```python
+   # Old
+   agent = MyAgent(self.next_id(), self, ...)
+
+   # New
+   agent = MyAgent(self, ...)
+   ```
+2. `Model.next_id()` is deprecated and will always return 0. Remove any calls to this method.
+3. `unique_id` is now unique relative to a Model instance and starts from 1.
+4. If you previously used custom `unique_id` values, you'll need to store that information in a separate attribute.
+5. Deprecation warning: Initializing an agent with two arguments (`unique_id` and `model`) will raise a warning. The `unique_id` argument will be ignored.
+
+- Ref: [PR #2226](https://github.com/projectmesa/mesa/pull/2226), [PR #2260](https://github.com/projectmesa/mesa/pull/2260), Mesa-examples [PR #194](https://github.com/projectmesa/mesa-examples/pull/194), [Issue #2213](https://github.com/projectmesa/mesa/issues/2213)
 
 
 ### AgentSet and `Model.agents`