Add ReadConfigFrom to load config from a struct

In cases where there isn't a configuration file, none of the fields on
the struct are bound to environment variables unless BindEnv was
explicitly called. AutomaticEnv essentially doesn't work with the config
file per spf13#761.

If you need to support this scenario, call ReadConfigFrom on the
destination struct before calling Unmarshal.

Signed-off-by: Carolyn Van Slyck <me@carolynvanslyck.com>

viper.go

@@ -995,6 +995,33 @@ func (v *Viper) Unmarshal(rawVal interface{}, opts ...DecoderConfigOption) error
 	return decode(v.AllSettings(), defaultDecoderConfig(rawVal, opts...))
 }
 
+// ReadConfigFrom applies the specified data to Viper's configuration.
+// Useful for setting defaults from the specified structs in cases
+// where a configuration file may not be present.
+func ReadConfigFrom(val interface{}) error {
+	return v.ReadConfigFrom(val)
+}
+
+// ReadConfigFrom applies the specified data to Viper's configuration.
+// Useful for setting defaults from the specified structs.
+func (v *Viper) ReadConfigFrom(val interface{}) error {
+	var tmp map[string]interface{}
+	err := mapstructure.Decode(val, &tmp)
+	if err != nil {
+		return fmt.Errorf("error decoding configuration from struct: %v", err)
+	}
+
+	cfgType := v.configType
+	defer func() { v.configType = cfgType }()
+	v.SetConfigType("json")
+	tmpJson, err := json.Marshal(tmp)
+	if err != nil {
+		return fmt.Errorf("error marshaling configuration from struct: %v", err)
+	}
+
+	return v.ReadConfig(bytes.NewReader(tmpJson))
+}
+
 // defaultDecoderConfig returns default mapsstructure.DecoderConfig with suppot
 // of time.Duration values & string slices
 func defaultDecoderConfig(output interface{}, opts ...DecoderConfigOption) *mapstructure.DecoderConfig {

viper_test.go

@@ -798,6 +798,37 @@ func TestUnmarshal(t *testing.T) {
 	)
 }
 
+// Check that we can unmarshal environment variables without
+// explicit bindings or a configuration file.
+func TestUnmarshalWithoutConfigFile(t *testing.T) {
+	v := New()
+	v.AutomaticEnv()
+
+	type Person struct {
+		Name string
+	}
+	type config struct {
+		Port   int
+		Person Person
+	}
+
+	testutil.Setenv(t, "PORT", "1313")
+
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	testutil.Setenv(t, "PERSON_NAME", "Sally")
+
+	C := config{}
+
+	v.ReadConfigFrom(C)
+
+	err := v.Unmarshal(&C)
+	if err != nil {
+		t.Fatalf("unable to decode into struct, %v", err)
+	}
+
+	assert.Equal(t, config{Port: 1313, Person: Person{Name: "Sally"}}, C)
+}
+
 func TestUnmarshalWithDecoderOptions(t *testing.T) {
 	Set("credentials", "{\"foo\":\"bar\"}")