diff --git a/drf_spectacular/openapi.py b/drf_spectacular/openapi.py
index 21f44082..7c89c27a 100644
--- a/drf_spectacular/openapi.py
+++ b/drf_spectacular/openapi.py
@@ -481,9 +481,10 @@ def _map_serializer_field(self, field, direction):
                 model_field = field.queryset.model._meta.pk
             else:
                 if isinstance(field.parent, serializers.ManyRelatedField):
-                    model_field = field.parent.parent.Meta.model._meta.pk
+                    relation_field = field.parent.parent.Meta.model._meta.get_field(field.parent.source)
                 else:
-                    model_field = field.parent.Meta.model._meta.pk
+                    relation_field = field.parent.Meta.model._meta.get_field(field.source)
+                model_field = relation_field.related_model._meta.pk
 
             # primary keys are usually non-editable (readOnly=True) and map_model_field correctly
             # signals that attribute. however this does not apply in the context of relations.
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index 8817ffb2..b879d406 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -1314,3 +1314,36 @@ def view_func(request, format=None):
     assert 'explode' in parameter
     assert 'style' in parameter
     assert parameter['schema']['type'] == 'array'
+
+
+def test_incorrect_foreignkey_type_on_readonly_field(no_warnings):
+    class ReferencingModel(models.Model):
+        id = models.UUIDField(primary_key=True)
+        referenced_model = models.ForeignKey(SimpleModel, on_delete=models.CASCADE)
+        referenced_model_ro = models.ForeignKey(SimpleModel, on_delete=models.CASCADE)
+        referenced_model_m2m = models.ManyToManyField(SimpleModel)
+        referenced_model_m2m_ro = models.ManyToManyField(SimpleModel)
+
+    class ReferencingModelSerializer(serializers.ModelSerializer):
+        indirect_referenced_model_ro = serializers.PrimaryKeyRelatedField(
+            source='referenced_model',
+            read_only=True,
+        )
+
+        class Meta:
+            fields = '__all__'
+            read_only_fields = ['id', 'referenced_model_ro', 'referenced_model_m2m_ro']
+            model = ReferencingModel
+
+    class ReferencingModelViewset(viewsets.ModelViewSet):
+        serializer_class = ReferencingModelSerializer
+        queryset = ReferencingModel.objects.all()
+
+    schema = generate_schema('/x/', ReferencingModelViewset)
+    properties = schema['components']['schemas']['ReferencingModel']['properties']
+
+    assert properties['referenced_model']['type'] == 'integer'
+    assert properties['referenced_model_ro']['type'] == 'integer'
+    assert properties['referenced_model_m2m']['items']['type'] == 'integer'
+    assert properties['referenced_model_m2m_ro']['items']['type'] == 'integer'
+    assert properties['indirect_referenced_model_ro']['type'] == 'integer'