register coercion from a Ore ring to its fraction field

src/sage/rings/polynomial/ore_function_element.py

@@ -11,7 +11,7 @@
 class OreFunction(AlgebraElement):
     def __init__(self, parent, numerator, denominator=None, simplify=True):
         AlgebraElement.__init__(self, parent)
-        ring = parent.ring()
+        ring = parent._ring
         numerator = ring(numerator)
         if denominator is None:
             denominator = ring.one()

src/sage/rings/polynomial/ore_function_field.py

@@ -56,7 +56,7 @@ def _coerce_map_from_base_ring(self):
 
     def _coerce_map_from_(self, P):
         if isinstance(P, OreFunctionField):
-            return P.ring().has_coerce_map_from(self._ring)
+            return P._ring.has_coerce_map_from(self._ring)
         if isinstance(P, Parent):
             return P.has_coerce_map_from(self._ring)
 
@@ -72,9 +72,6 @@ def _repr_(self):
             s += derivation._repr_()
         return s
 
-    def ring(self):
-        return self._ring
-
     def change_var(self, var):
         return OreFunctionField(self._ring.change_var(var)) 
 

src/sage/rings/polynomial/ore_polynomial_ring.py

@@ -1058,14 +1058,15 @@ def is_commutative(self):
     def fraction_field(self):
         from sage.rings.polynomial.ore_function_field import OreFunctionField
         if self.base_ring() in Fields():
-            ring = self
+            return OreFunctionField(self)
+        base = self.base_ring().fraction_field()
+        if self._derivation is None:
+            twist = self._morphism.extend_to_fraction_field()
         else:
-            base = self.base_ring().fraction_field()
-            if self._derivation is None:
-                twist = self._morphism.extend_to_fraction_field()
-            else:
-                twist = self._derivation.extend_to_fraction_field()
-            name = self.variable_name()
-            sparse = self.is_sparse()
-            ring = OrePolynomialRing(base, twist, name, sparse)
-        return OreFunctionField(ring)
+            twist = self._derivation.extend_to_fraction_field()
+        name = self.variable_name()
+        sparse = self.is_sparse()
+        ring = OrePolynomialRing(base, twist, name, sparse)
+        frac = OreFunctionField(ring)
+        frac.register_coercion(self)
+        return frac