documented async model form's validation and constructor

Author: amirreza8002

docs/forms/model_form.md

@@ -29,6 +29,67 @@ class AuthorForm(AsyncModelForm):
     fields = ("name",)
 ```
 
+#### Construction
+
+normally you instantiate a field like this:
+`form = AuthorForm(data)`
+and that works fine
+
+but sometimes you pass in an instance to the form (e.g: you are building an update view)
+```pycon
+>>> author = Author.objects.get(pk=1)
+>>> form = AuthorForm(instance=author)
+```
+
+in this case, you will probably face an error saying you can't do synchronous operations in an async environment
+for situations like this, we provide an alternative contractor called `from_async()`
+
+```pycon
+author = await Author.objects.aget(pk=1)
+form = await AuthorForm.from_async(instance=author)
+```
+
+note that `from_async()` is an async method that should be `await`ed
+
+#### validation
+if a validator requires database access (e.g: unique validator) you will face a problem in async views
+to solve that, we provide a few methods
+
+* ais_valid
+```python
+if await form.ais_valid():
+    # handle data
+```
+an awaitable version of django's [Form.is_valid](https://docs.djangoproject.com/en/5.1/ref/forms/api/#django.forms.Form.is_valid)
+
+* aerrors (property)
+```pycon
+>>> errors = await form.aerrors
+```
+an awaitable version of django's [Form.errors](https://docs.djangoproject.com/en/5.1/ref/forms/api/#django.forms.Form.errors)
+
+* afull_clean
+```python
+await form.afull_clean()
+```
+an awaitable version of django's `Form.full_clean`, you typically don't call this manually.
+
+
+
+#### Manual form rendering
+django's templating system is fully sync, so you can't call async methods and functions in the templates
+you can call sync methods and render the page using `asgiref.sync.sync_to_async`, or you can gather the data in your view and pass them in as context.
+
+a few methods have been provided so forms can be rendered manually:
+
+1. arender: an `await`able version of django's [Form.render](https://docs.djangoproject.com/en/5.1/ref/forms/api/#render)
+2. aas_div: an `await`able version of django's [Form.as_div](https://docs.djangoproject.com/en/5.1/ref/forms/api/#as-div)
+3. aas_p: an `await`able version of django's [Form.as_p](https://docs.djangoproject.com/en/5.1/ref/forms/api/#django.forms.Form.as_p)
+4. aas_ul: an `await`able version of django's [Form.as_ul](https://docs.djangoproject.com/en/5.1/ref/forms/api/#as-ul)
+5. aas_table: an `await`able version of django's [Form.as_table](https://docs.djangoproject.com/en/5.1/ref/forms/api/#as-table)
+
+
+note that the sync versions are still available.
 
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