Add ability to define event properties across all events (#63)

* adding new config prop Config.DefaultProperties

* change config name from DefaultProperties to DefaultEventProperties

* better config description

config.go

@@ -52,6 +52,11 @@ type Config struct {
 	// `os.Stderr`.
 	Logger Logger
 
+	// Properties that will be included in every event sent by the client.
+	// This is useful for adding common metadata like service name or app version across all events.
+	// If a property conflict occurs, the value from DefaultEventProperties will overwrite any existing value.
+	DefaultEventProperties Properties
+
 	// The callback object that will be used by the client to notify the
 	// application when messages sends to the backend API succeeded or failed.
 	Callback Callback

fixtures/test-merge-capture.json

@@ -0,0 +1,22 @@
+{
+  "api_key": "Csyjlnlun3OzyNJAafdlv",
+  "batch": [
+    {
+      "distinct_id": "123456",
+      "event": "Download",
+      "library": "posthog-go",
+      "library_version": "1.0.0",
+      "properties": {
+        "$lib": "posthog-go",
+        "$lib_version": "1.0.0",
+        "application": "PostHog Go",
+        "platform": "macos",
+        "service": "api",
+        "version": "1.0.0"
+      },
+      "send_feature_flags": false,
+      "timestamp": "2015-07-10T23:00:00Z",
+      "type": "capture"
+    }
+  ]
+}

posthog.go

@@ -227,6 +227,7 @@ func (c *client) Enqueue(msg Message) (err error) {
 			}
 			m.Properties["$active_feature_flags"] = featureKeys
 		}
+		m.Properties.Merge(c.DefaultEventProperties)
 		c.setLastCapturedEvent(m)
 		msg = m
 

posthog_test.go

@@ -436,6 +436,40 @@ func TestCaptureWithTimestamp(t *testing.T) {
 	}
 }
 
+func TestCaptureWithDefaultProperties(t *testing.T) {
+	var ref = strings.TrimSpace(fixture("test-merge-capture.json"))
+
+	body, server := mockServer()
+	defer server.Close()
+
+	client, _ := NewWithConfig("Csyjlnlun3OzyNJAafdlv", Config{
+		Endpoint:               server.URL,
+		Verbose:                true,
+		DefaultEventProperties: NewProperties().Set("service", "api"),
+		Logger:                 t,
+		BatchSize:              1,
+		now:                    mockTime,
+		uid:                    mockId,
+	})
+	defer client.Close()
+
+	client.Enqueue(Capture{
+		Event:      "Download",
+		DistinctId: "123456",
+		Properties: Properties{
+			"application": "PostHog Go",
+			"version":     "1.0.0",
+			"platform":    "macos", // :)
+		},
+		SendFeatureFlags: false,
+		Timestamp:        time.Date(2015, time.July, 10, 23, 0, 0, 0, time.UTC),
+	})
+
+	if res := string(<-body); ref != res {
+		t.Errorf("invalid response:\n- expected %s\n- received: %s", ref, res)
+	}
+}
+
 func TestCaptureMany(t *testing.T) {
 	var ref = strings.TrimSpace(fixture("test-many-capture.json"))
 

properties.go

@@ -22,3 +22,13 @@ func (p Properties) Set(name string, value interface{}) Properties {
 	p[name] = value
 	return p
 }
+
+// Merge adds the properties from the provided `props` into the receiver `p`.
+// If a property in `props` already exists in `p`, its value will be overwritten.
+func (p Properties) Merge(props Properties) Properties {
+	for k, v := range props {
+		p[k] = v
+	}
+
+	return p
+}

properties_test.go

@@ -34,5 +34,17 @@ func TestPropertiesMulti(t *testing.T) {
 	if !reflect.DeepEqual(p0, p1) {
 		t.Errorf("invalid properties produced by chained setters:\n- expected %#v\n- found: %#v", p0, p1)
 	}
+}
+
+func TestPropertiesMerge(t *testing.T) {
+	defaultProps := Properties{"currency": "USD", "service": "api"}
+
+	props := NewProperties().Set("title", "A").Set("value", 0.5).Set("currency", "BRL")
+	props.Merge(defaultProps)
 
+	expected := Properties{"title": "A", "value": 0.5, "currency": "USD", "service": "api"}
+
+	if !reflect.DeepEqual(props, Properties{"title": "A", "value": 0.5, "currency": "USD", "service": "api"}) {
+		t.Errorf("invalid properties produced by merge:\n- expected %#v\n- found: %#v", expected, props)
+	}
 }