Add common utility functions

The process of fetching a distantly related field type (for the purpose
of verifying its existence/validity or for actually getting the type
itself) was repeated in a few places in the project code.

datatableview/utils.py

@@ -2,6 +2,7 @@
 
 from django.utils.encoding import StrAndUnicode
 from django.template.loader import render_to_string
+from django.db.models.fields import FieldDoesNotExist
 
 # Sane boundary constants
 MINIMUM_PAGE_LENGTH = 5
@@ -31,6 +32,40 @@
     'sort_column_direction': 'sSortDir_%d',
 }
 
+def resolve_orm_path(model, orm_path):
+    """
+    Follows the queryset-style query path of ``orm_path`` starting from ``model`` class.  If the
+    path ends up referring to a bad field name, ``django.db.models.fields.FieldDoesNotExist`` will
+    be raised.
+
+    """
+
+    bits = orm_path.split('__')
+    endpoint_model = reduce(get_model_at_related_field, [model] + bits[:-1])
+    field, _, _, _ = endpoint_model._meta.get_field_by_name(bits[-1])
+    return field
+
+def get_model_at_related_field(model, attr):
+    """
+    Looks up ``attr`` as a field of ``model`` and returns the related model class.  If ``attr`` is
+    not a relationship field, ``ValueError`` is raised.
+
+    """
+
+    try:
+        field, _, direct, m2m = model._meta.get_field_by_name(attr)
+    except FieldDoesNotExist:
+        raise
+
+    if m2m:
+        model = field.field.rel.to
+    elif direct:
+        model = field.related.model
+    else:
+        raise ValueError("{}.{} ({}) is not a relationship field.".format(model.__name__, attr,
+                field.__class__.__name__))
+    return model
+
 class DatatableStructure(StrAndUnicode):
     """
     A class designed to be echoed directly to into template HTML to represent a skeleton table