Sketched out a converter registry for model -> storymarket conversion.

django_storymarket/admin.py

@@ -10,6 +10,7 @@
 
 import storymarket
 
+from . import converters
 from .models import SyncedObject
 
 def attrs(**kwargs):
@@ -37,6 +38,9 @@ def __init__(self, model, admin_site):
         self.list_display = list_display
 
         self.storymarket = storymarket.Storymarket(settings.STORYMARKET_API_KEY)
+
+        # FIXME: this should be elsewhere.
+        converters.autodiscover()
 
     @attrs(short_description='Upload selected %(verbose_name_plural)s to Storymarket')
     def upload_to_storymarket(self, request, queryset):
@@ -46,7 +50,7 @@ def upload_to_storymarket(self, request, queryset):
             # The user has confirumed the uploading.
             num_uploaded = 0
             for obj in queryset:
-                sm_data = self.to_storymarket(obj)
+                sm_data = converters.convert(self.storymarket, obj)
                 sm_type = sm_data.pop('type')
                 manager = getattr(self.storymarket, sm_type)
                 sm_obj = manager.create(sm_data)
@@ -61,7 +65,7 @@ def upload_to_storymarket(self, request, queryset):
         # Generate a list of converted objects to "preview" as an upload.
         # These is a list-of-dicts for template convienience
         previewed_objects = [
-            {'object': obj, 'preview': self.to_storymarket(obj)}
+            {'object': obj, 'preview': converters.convert(self.storymarket, obj)}
             for obj in queryset
         ]
 

django_storymarket/converters.py

@@ -0,0 +1,127 @@
+"""
+Converters are the glue that converts a Django model into a dict of data to be
+posted to Storymarket. This module implements a registry for them.
+
+Converter functions are simple functions that take two arguments -- the
+Storymarket API object and the model instance to convert -- and return
+a dictionary of data to be used for the upload. This dict should be in
+the simplified format accepted by the Storymarket API, and should have
+one extra key: the type of data to be uploaded.
+
+For example::
+
+    def story_to_storymarket(api, obj):
+        return {
+            "type": "text",
+            "title": obj.headline,
+            "author": "jacobian",
+            "org": api.orgs.get(MY_ORG_ID),
+            "category": api.subcategories.get(SOME_CATEGORY_ID),
+            "content": obj.body
+        }
+
+For binary types, the returned dict should have a ``blob`` key; the value can
+either be the binary data as a string or (more likely) a file-like object::
+
+    def image_to_storymarket(api, obj):
+        return {
+            "type": "photos",
+            ...
+            "blob": obj.image_field.open(),
+        }
+"""
+
+from django.conf import settings
+from django.utils.importlib import import_module
+from django.utils.module_loading import module_has_submodule
+
+_registry = {}
+_FALLBACK_KEY = '*'
+_CONVERTER_MODULE_NAME = 'storymarket_converters'
+
+def convert(api, instance):
+    """
+    Convert a model instance using its registered converter.
+    
+    If it fails, this'll raise :exc:`CannotConvert`.
+    
+    :param api: The storymarket API instance.
+    :param instance: The model instance to convert.
+    :rtype: dict
+    """
+    registry_key = str(instance._meta)
+
+    # I'm using look-before-you-leap instead of better-to-ask-for-permission
+    # here because a try/except might accidentally catch a KeyError raised
+    # by the converter itself.
+    if registry_key in _registry:
+        return _registry[registry_key](api, instance)
+    elif _FALLBACK_KEY in _registry:
+        return _registry[_FALLBACK_KEY](api, instance)
+    else:
+        raise CannotConvert("Can't convert %s objects." % instance._meta)
+
+class CannotConvert(Exception):
+    pass
+
+def register(model, callback):
+    """
+    Register a converter.
+    
+    :param model: The model class to register the converter for.
+    :param callback: The conversion function.
+    """
+    _registry[str(model._meta)] = callback
+
+def register_fallback_converter(callback):
+    """
+    Register a fallback converter to be used if a model-specific converter
+    doesn't already exist.
+    
+    This converter should either convert an object or raise :exc:`CannotConvert`.
+    
+    :param callback: The conversion function.
+    """
+    _registry[_FALLBACK_KEY] = callback
+
+def unregister(model):
+    """
+    Remove a converter for a model, if registered.
+    
+    :param model: The model class to unregister.
+    """
+    try:
+        del _registry[str(model._meta)]
+    except KeyError:
+        pass
+
+def unregister_fallback_converter():
+    """
+    Remove a fallback converter, if registered.
+    """
+    try:
+        del _registry[_FALLBACK_KEY]
+    except KeyError:
+        pass
+
+_discovery_done = False
+def autodiscover():
+    """
+    Auto-discover converter modules from settings.INSTALLED_APPS.
+    
+    This code is cribbed from admin.autodiscover.
+    """
+    global _discovery_done
+    if _discovery_done: return
+
+    for app in settings.INSTALLED_APPS:
+        mod = import_module(app)
+        try:
+            import_module("%s.%s" % (app, _CONVERTER_MODULE_NAME))
+        except:
+            # Ignore the error if the app just doesn't have a converter
+            # module, but bubble it up if it was anything else.
+            if module_has_submodule(mod, _CONVERTER_MODULE_NAME):
+                raise
+
+    _discovery_done = True

example/admin.py

@@ -5,14 +5,6 @@
 from .models import ExampleStory
 
 class ExampleStoryAdmin(StorymarketAdmin, admin.ModelAdmin):
-    def to_storymarket(self, obj):
-        return {
-            "type": "text",
-            "title": obj.headline,
-            "author": "jacobian",
-            "org": self.storymarket.orgs.get(12),
-            "category": self.storymarket.subcategories.get(12),
-            "content": obj.body
-        }
+    pass
 
 admin.site.register(ExampleStory, ExampleStoryAdmin)

example/storymarket_converters.py

@@ -0,0 +1,16 @@
+from __future__ import absolute_import
+
+from .models import ExampleStory
+import django_storymarket.converters
+
+def story_to_storymarket(api, obj):
+    return {
+        "type": "text",
+        "title": obj.headline,
+        "author": "jacobian",
+        "org": api.orgs.get(12),
+        "category": api.subcategories.get(12),
+        "content": obj.body
+    }
+
+django_storymarket.converters.register(ExampleStory, story_to_storymarket)