Serialize links for books

bookwyrm/models/link.py

@@ -23,11 +23,15 @@ class Link(ActivitypubMixin, BookWyrmModel):
             del kwargs["broadcast"]
         return super().save(*args, **kwargs)
 
+    def to_activity(self, omit=(), **kwargs):
+        """we don't need ALL the fields"""
+        return super().to_activity(omit=("@context", "id"), **kwargs)
+
 
 class FileLink(Link):
     """a link to a file"""
 
-    book = fields.ForeignKey(
+    book = models.ForeignKey(
         "Book", on_delete=models.CASCADE, related_name="file_links", null=True
     )
     filetype = fields.CharField(max_length=5, activitypub_field="mediaType")