helper/schema: don't mark things computed if an ID is set

helper/schema/schema.go

@@ -401,9 +401,14 @@ func (m schemaMap) diffString(
 	}
 
 	if os == ns {
-		// They're the same value, return no diff as long as we're not
-		// computing a new value.
-		if os != "" || !schema.Computed {
+		// They're the same value. If there old value is not blank or we
+		// have an ID, then return right away since we're already setup.
+		if os != "" || d.Id() != "" {
+			return nil
+		}
+
+		// Otherwise, only continue if we're computed
+		if !schema.Computed {
 			return nil
 		}
 	}

helper/schema/schema_test.go

@@ -76,6 +76,27 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"availability_zone": &Schema{
+					Type:     TypeString,
+					Optional: true,
+					Computed: true,
+					ForceNew: true,
+				},
+			},
+
+			State: &terraform.ResourceState{
+				ID: "foo",
+			},
+
+			Config: map[string]interface{}{},
+
+			Diff: nil,
+
+			Err: false,
+		},
+
 		// String with StateFunc
 		{
 			Schema: map[string]*Schema{