Function to check nil Identifier (#97)

* Validation function for resource Identifier

* Renamed: Valid -> Nil in order to avoid confusions

rpc/resource/README.md

@@ -31,3 +31,27 @@ message MyMessage {
 
 Please give a read to [README](../../gorm/resource/README.md) of `gorm/resource`
 package to see how it could be used with gorm and `protoc-gen-gorm` generated code. 
+
+# How to check that an Identifier is nil?
+
+In order to check that an identifier is nil use `resource.Nil` function.
+
+```go
+package main
+
+import (
+	"fmt"
+
+	"github.com/infobloxopen/atlas-app-toolkit/rpc/resource"
+)
+
+func main() {
+    var id *resource.Identifier
+
+    if resource.Nil(id) {
+    	fmt.Println("resource is nil identifier")
+    }
+}
+```
+
+See [Unit test](nil_test.go).

rpc/resource/nil.go

@@ -0,0 +1,17 @@
+package resource
+
+// String method calls proto.CompactTextString which returns "<nil>" string
+// in case if passed proto.Message is nil.
+// The "<nil>" string is common representation of nil value in Go code. See fmt package.
+const pbnil = "<nil>"
+
+// Nil reports whether id is empty identifier or not.
+// The id is empty if it is either nil or could be converted to the empty string by its String method.
+func Nil(id *Identifier) (ok bool) {
+	// comparison with pbnil is mostly paranoid check
+	// should never happen
+	if id == nil || id.String() == "" || id.String() == pbnil {
+		return true
+	}
+	return
+}

rpc/resource/nil_test.go

@@ -0,0 +1,29 @@
+package resource
+
+import "testing"
+
+func TestNil(t *testing.T) {
+	tcases := []struct {
+		Identifier *Identifier
+		Expected   bool
+	}{
+		{
+			Identifier: nil,
+			Expected:   true,
+		},
+		{
+			Identifier: &Identifier{},
+			Expected:   true,
+		},
+		{
+			Identifier: &Identifier{ResourceId: "uuid"},
+			Expected:   false,
+		},
+	}
+
+	for n, tc := range tcases {
+		if v := Nil(tc.Identifier); v != tc.Expected {
+			t.Errorf("%d: invalid result %t, expected %t", n, v, tc.Expected)
+		}
+	}
+}