diff --git a/django/db/backends/util.py b/django/db/backends/util.py
index 7228b4046b..f9c07a9741 100644
--- a/django/db/backends/util.py
+++ b/django/db/backends/util.py
@@ -124,4 +124,9 @@ def format_number(value, max_digits, decimal_places):
     Formats a number into a string with the requisite number of digits and
     decimal places.
     """
-    return u"%.*f" % (decimal_places, value)
+    if isinstance(value, decimal.Decimal):
+        context = decimal.getcontext().copy()
+        context.prec = max_digits
+        return u'%s' % str(value.quantize(decimal.Decimal(".1") ** decimal_places, context=context))
+    else:
+        return u"%.*f" % (decimal_places, value)
diff --git a/tests/regressiontests/model_fields/models.py b/tests/regressiontests/model_fields/models.py
index 455e2b3ded..a1c2bb0bc8 100644
--- a/tests/regressiontests/model_fields/models.py
+++ b/tests/regressiontests/model_fields/models.py
@@ -32,6 +32,9 @@ class Whiz(models.Model):
         (0,'Other'),
     )
     c = models.IntegerField(choices=CHOICES, null=True)
+    
+class BigD(models.Model):
+    d = models.DecimalField(max_digits=38, decimal_places=30)
 
 __test__ = {'API_TESTS':"""
 # Create a couple of Places.
@@ -78,5 +81,11 @@ u''
 >>> Foo.objects.filter(d=u'1.23')
 []
 
-
+# Regression test for #5079 -- ensure decimals don't go through a corrupting
+# float conversion during save.  
+>>> bd = BigD(d="12.9")
+>>> bd.save()
+>>> bd = BigD.objects.get(pk=bd.pk)
+>>> bd.d == decimal.Decimal("12.9")
+True
 """}