Ported over Field.deconstruct() from my schema alteration branch.

This is to help other ongoing branches which would benefit from
this functionality.

django/db/models/fields/files.py

@@ -227,6 +227,17 @@ class FileField(Field):
         kwargs['max_length'] = kwargs.get('max_length', 100)
         super(FileField, self).__init__(verbose_name, name, **kwargs)
 
+    def deconstruct(self):
+        name, path, args, kwargs = super(FileField, self).deconstruct()
+        if kwargs.get("max_length", None) != 100:
+            kwargs["max_length"] = 100
+        else:
+            del kwargs["max_length"]
+        kwargs['upload_to'] = self.upload_to
+        if self.storage is not default_storage:
+            kwargs['storage'] = self.storage
+        return name, path, args, kwargs
+
     def get_internal_type(self):
         return "FileField"
 
@@ -326,6 +337,14 @@ class ImageField(FileField):
         self.width_field, self.height_field = width_field, height_field
         super(ImageField, self).__init__(verbose_name, name, **kwargs)
 
+    def deconstruct(self):
+        name, path, args, kwargs = super(ImageField, self).deconstruct()
+        if self.width_field:
+            kwargs['width_field'] = self.width_field
+        if self.height_field:
+            kwargs['height_field'] = self.height_field
+        return name, path, args, kwargs
+
     def contribute_to_class(self, cls, name):
         super(ImageField, self).contribute_to_class(cls, name)
         # Attach update_dimension_fields so that dimension fields declared

django/db/models/fields/related.py

@@ -1151,6 +1151,27 @@ class ForeignKey(ForeignObject):
         )
         super(ForeignKey, self).__init__(to, ['self'], [to_field], **kwargs)
 
+    def deconstruct(self):
+        name, path, args, kwargs = super(ForeignKey, self).deconstruct()
+        # Handle the simpler arguments
+        if self.db_index:
+            del kwargs['db_index']
+        else:
+            kwargs['db_index'] = False
+        if self.db_constraint is not True:
+            kwargs['db_constraint'] = self.db_constraint
+        if self.rel.on_delete is not CASCADE:
+            kwargs['on_delete'] = self.rel.on_delete
+        # Rel needs more work.
+        rel = self.rel
+        if self.rel.field_name:
+            kwargs['to_field'] = self.rel.field_name
+        if isinstance(self.rel.to, basestring):
+            kwargs['to'] = self.rel.to
+        else:
+            kwargs['to'] = "%s.%s" % (self.rel.to._meta.app_label, self.rel.to._meta.object_name)
+        return name, path, args, kwargs
+
     @property
     def related_field(self):
         return self.foreign_related_fields[0]
@@ -1268,6 +1289,12 @@ class OneToOneField(ForeignKey):
         kwargs['unique'] = True
         super(OneToOneField, self).__init__(to, to_field, OneToOneRel, **kwargs)
 
+    def deconstruct(self):
+        name, path, args, kwargs = super(OneToOneField, self).deconstruct()
+        if "unique" in kwargs:
+            del kwargs['unique']
+        return name, path, args, kwargs
+
     def contribute_to_related_class(self, cls, related):
         setattr(cls, related.get_accessor_name(),
                 SingleRelatedObjectDescriptor(related))
@@ -1357,6 +1384,21 @@ class ManyToManyField(RelatedField):
 
         super(ManyToManyField, self).__init__(**kwargs)
 
+    def deconstruct(self):
+        name, path, args, kwargs = super(ManyToManyField, self).deconstruct()
+        # Handle the simpler arguments
+        if self.rel.db_constraint is not True:
+            kwargs['db_constraint'] = self.db_constraint
+        if "help_text" in kwargs:
+            del kwargs['help_text']
+        # Rel needs more work.
+        rel = self.rel
+        if isinstance(self.rel.to, basestring):
+            kwargs['to'] = self.rel.to
+        else:
+            kwargs['to'] = "%s.%s" % (self.rel.to._meta.app_label, self.rel.to._meta.object_name)
+        return name, path, args, kwargs
+
     def _get_path_info(self, direct=False):
         """
         Called by both direct an indirect m2m traversal.