allow jsonapi type customization

jsonapi.go

@@ -385,3 +385,27 @@ type MarshalIdentifier interface {
 type UnmarshalIdentifier interface {
 	UnmarshalID(id string) error
 }
+
+// MarshalType can be optionally implemented to control marshaling of the type field.
+//
+// The order of operations for marshaling the type field is:
+//
+//  1. Use MarshalType if it is implemented
+//  2. Use the value from the jsonapi tag on the primary field
+//  3. Fail
+type MarshalType interface {
+	MarshalType() string
+}
+
+// UnmarshalType can be optionally implemented to control unmarshaling of the type field from a string.
+// Since the type is not typically set as a field on the object, this is an opportunity to return an error
+// if the passed in type from the payload is unexpected. This allows customization of the expected type field.
+//
+// The order of operations for checking the type field is:
+//
+//  1. Use UnmarshalType if it is implemented, fail if it returns an error
+//  2. Compare against the type provided in the jsonapi tag on the primary field
+//  3. Fail
+type UnmarshalType interface {
+	UnmarshalType(jsonapiType string) error
+}

jsonapi_test.go

@@ -56,6 +56,7 @@ var (
 	articleLinkedInvalidSelfMeta                 = ArticleLinkedInvalidSelfMeta{ID: "1"}
 	articleOmitTitleFull                         = ArticleOmitTitle{ID: "1"}
 	articleOmitTitlePartial                      = ArticleOmitTitle{ID: "1", Subtitle: "A"}
+	articleACustomType                           = ArticleTypeOverride{ID: "1", Title: "A"}
 	articleAIntID                                = ArticleIntID{ID: 1, Title: "A"}
 	articleBIntID                                = ArticleIntID{ID: 2, Title: "B"}
 	articlesIntIDABPtr                           = []*ArticleIntID{&articleAIntID, &articleBIntID}
@@ -150,6 +151,7 @@ var (
 	emptySingleBody                       = `{"data":{}}`
 	emptyManyBody                         = `{"data":[]}`
 	articleABody                          = `{"data":{"type":"articles","id":"1","attributes":{"title":"A"}}}`
+	articleACustomTypeBody                = `{"data":{"type":"customarticles","id":"1","attributes":{"title":"A"}}}`
 	articleANoIDBody                      = `{"data":{"type":"articles","attributes":{"title":"A"}}}`
 	articleAInvalidTypeBody               = `{"data":{"type":"not-articles","id":"1","attributes":{"title":"A"}}}`
 	articleOmitTitleFullBody              = `{"data":{"type":"articles","id":"1"}}`
@@ -352,6 +354,22 @@ type ArticleIntID struct {
 	Title string `jsonapi:"attribute" json:"title"`
 }
 
+type ArticleTypeOverride struct {
+	ID    string `jsonapi:"primary,articles"`
+	Title string `jsonapi:"attribute" json:"title"`
+}
+
+func (a *ArticleTypeOverride) MarshalType() string {
+	return "customarticles"
+}
+
+func (a *ArticleTypeOverride) UnmarshalType(jsonapiType string) error {
+	if jsonapiType != "customarticles" {
+		return fmt.Errorf("invalid type: %s", jsonapiType)
+	}
+	return nil
+}
+
 func (a *ArticleIntID) MarshalID() string {
 	return fmt.Sprintf("%d", a.ID)
 }

marshal.go

@@ -355,6 +355,9 @@ func (d *document) makeResourceObject(v any, vt reflect.Type, m *Marshaler) (*re
 		switch tag.directive {
 		case primary:
 			ro.Type = tag.resourceType
+			if vmt, ok := v.(MarshalType); ok {
+				ro.Type = vmt.MarshalType()
+			}
 			if !isValidMemberName(ro.Type, m.memberNameValidationMode) {
 				// type names count as member names
 				return nil, &MemberNameValidationError{ro.Type}

marshal_test.go

@@ -158,6 +158,11 @@ func TestMarshal(t *testing.T) {
 			given:       []string{"a", "b"},
 			expect:      "",
 			expectError: &TypeError{Actual: "string", Expected: []string{"struct"}},
+		}, {
+			description: "*ArticleMarshalType (MarshalType)",
+			given:       &articleACustomType,
+			expect:      articleACustomTypeBody,
+			expectError: nil,
 		}, {
 			description: "*ArticleIntID (MarshalIdentifier)",
 			given:       &articleAIntID,

unmarshal.go

@@ -3,6 +3,7 @@ package jsonapi
 import (
 	"encoding"
 	"encoding/json"
+	"errors"
 	"reflect"
 )
 
@@ -232,7 +233,17 @@ func (ro *resourceObject) unmarshalFields(v any, rv reflect.Value, rt reflect.Ty
 			if setPrimary {
 				return ErrUnmarshalDuplicatePrimaryField
 			}
-			if ro.Type != jsonapiTag.resourceType {
+
+			if vut, ok := v.(UnmarshalType); ok {
+				err := vut.UnmarshalType(ro.Type)
+				if err != nil {
+					return errors.Join(
+						&TypeError{Actual: ro.Type, Expected: []string{jsonapiTag.resourceType}},
+						err,
+					)
+				}
+
+			} else if ro.Type != jsonapiTag.resourceType {
 				return &TypeError{Actual: ro.Type, Expected: []string{jsonapiTag.resourceType}}
 			}
 			if !isValidMemberName(ro.Type, m.memberNameValidationMode) {

unmarshal_test.go

@@ -149,6 +149,16 @@ func TestUnmarshal(t *testing.T) {
 			},
 			expect:      []*Article{&articleA, &articleB},
 			expectError: nil,
+		}, {
+			description: "*ArticleUnmarshalType",
+			given:       articleACustomTypeBody,
+			do: func(body []byte) (any, error) {
+				var a ArticleTypeOverride
+				err := Unmarshal(body, &a)
+				return &a, err
+			},
+			expect:      &articleACustomType,
+			expectError: nil,
 		}, {
 			description: "*ArticleIntID",
 			given:       articleABody,