make media_manager a property of MediaEntry in mixin.py

In all cases where get_media_manager(_media_type_as_string) was called in
our code base we ultimately passed in a "MediaEntry().media_type" to get
the matching MEDIA_MANAGER. It so makes sense to make this a function of
the MediaEntry rather than a global function in mediagoblin.media_types and
passing around media_entry.media_type as arguments all the time.

It saves a few import statements and arguments. I also made it so the
Media_manager property is cached for subsequent calls, although I am not too
sure that this is needed (there are other cases for which this would make
more sense)

Also add a get_media_manager test to the media submission tests. It submits
an image and checks that both media.media_type and media.media_manager
return the right thing. Not sure if these tests could not be merged with an
existing submission test, but it can't hurt to have things explicit.

TODO: Right now we iterate through all existing media_managers to find the
right one based on the string of its module name. This should be made a simple
dict lookup to avoid all the extra work.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/db/mixin.py

@@ -27,9 +27,11 @@ These functions now live here and get "mixed in" into the
 real objects.
 """
 
+from werkzeug.utils import cached_property
+
 from mediagoblin import mg_globals
 from mediagoblin.auth import lib as auth_lib
-from mediagoblin.media_types import get_media_manager
+from mediagoblin.media_types import get_media_managers, FileTypeNotSupported
 from mediagoblin.tools import common, licenses
 from mediagoblin.tools.text import cleaned_markdown_conversion
 from mediagoblin.tools.url import slugify
@@ -99,6 +101,7 @@ class MediaEntryMixin(object):
     def slug_or_id(self):
         return (self.slug or self._id)
 
+
     def url_for_self(self, urlgen, **extra_args):
         """
         Generate an appropriate url for ourselves
@@ -125,11 +128,26 @@ class MediaEntryMixin(object):
         else:
             # No thumbnail in media available. Get the media's
             # MEDIA_MANAGER for the fallback icon and return static URL
-            # Raise FileTypeNotSupported in case no such manager is enabled
-            manager = get_media_manager(self.media_type)
+            # Raises FileTypeNotSupported in case no such manager is enabled
+            manager = self.media_manager
             thumb_url = mg_globals.app.staticdirector(manager[u'default_thumb'])
         return thumb_url
 
+    @cached_property
+    def media_manager(self):
+        """Returns the MEDIA_MANAGER of the media's media_type
+
+        Raises FileTypeNotSupported in case no such manager is enabled
+        """
+        # TODO, we should be able to make this a simple lookup rather
+        # than iterating through all media managers.
+        for media_type, manager in get_media_managers():
+            if media_type == self.media_type:
+                return manager
+        # Not found?  Then raise an error
+        raise FileTypeNotSupported(
+            "MediaManager not in enabled types.  Check media_types in config?")
+
     def get_fail_exception(self):
         """
         Get the exception that's appropriate for this error