solution

src/main/java/mate/academy/mapstruct/config/MapperConfig.java

@@ -0,0 +1,13 @@
+package mate.academy.mapstruct.config;
+
+import org.mapstruct.InjectionStrategy;
+import org.mapstruct.NullValueCheckStrategy;
+
+@org.mapstruct.MapperConfig(
+        componentModel = "spring",
+        injectionStrategy = InjectionStrategy.CONSTRUCTOR,
+        nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS,
+        implementationPackage = "<PACKAGE_NAME>.impl"
+)
+public class MapperConfig {
+}

src/main/java/mate/academy/mapstruct/mapper/GroupMapper.java

@@ -1,11 +1,24 @@
 package mate.academy.mapstruct.mapper;
 
+import java.util.Optional;
+import mate.academy.mapstruct.config.MapperConfig;
 import mate.academy.mapstruct.dto.group.CreateGroupRequestDto;
 import mate.academy.mapstruct.dto.group.GroupDto;
 import mate.academy.mapstruct.model.Group;
+import org.mapstruct.Mapper;
+import org.mapstruct.Named;
 
+@Mapper(config = MapperConfig.class)
 public interface GroupMapper {
+
     GroupDto toDto(Group group);
 
     Group toModel(CreateGroupRequestDto requestDto);
+
+    @Named("groupById")
+    default Group groupById(Long id) {
+        return Optional.ofNullable(id)
+                .map(Group::new)
+                .orElse(null);
+    }
 }

src/main/java/mate/academy/mapstruct/mapper/StudentMapper.java

@@ -1,14 +1,45 @@
 package mate.academy.mapstruct.mapper;
 
+import java.util.List;
+import mate.academy.mapstruct.config.MapperConfig;
 import mate.academy.mapstruct.dto.student.CreateStudentRequestDto;
 import mate.academy.mapstruct.dto.student.StudentDto;
 import mate.academy.mapstruct.dto.student.StudentWithoutSubjectsDto;
 import mate.academy.mapstruct.model.Student;
+import mate.academy.mapstruct.model.Subject;
+import org.mapstruct.AfterMapping;
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.MappingTarget;
 
+@Mapper(config = MapperConfig.class, uses = GroupMapper.class)
 public interface StudentMapper {
+    @Mapping(source = "group.id", target = "groupId")
+    @Mapping(target = "subjectIds", ignore = true)
     StudentDto toDto(Student student);
 
+    @AfterMapping
+    default void setSubjectIds(@MappingTarget StudentDto studentDto, Student student) {
+        if (student.getSubjects() != null) {
+            List<Long> subjectIds = student.getSubjects().stream()
+                    .map(Subject::getId)
+                    .toList();
+            studentDto.setSubjectIds(subjectIds);
+        }
+    }
+
+    @Mapping(target = "groupId", source = "group.id")
     StudentWithoutSubjectsDto toStudentWithoutSubjectsDto(Student student);
 
+    @Mapping(target = "group", source = "groupId", qualifiedByName = "groupById")
+    @Mapping(target = "subjects", ignore = true)
     Student toModel(CreateStudentRequestDto requestDto);
+
+    @AfterMapping
+    default void setSubjects(@MappingTarget Student student, CreateStudentRequestDto studentDto) {
+        List<Subject> subjects = studentDto.subjects().stream()
+                .map(Subject::new)
+                .toList();
+        student.setSubjects(subjects);
+    }
 }

src/main/java/mate/academy/mapstruct/mapper/SubjectMapper.java

@@ -1,9 +1,12 @@
 package mate.academy.mapstruct.mapper;
 
+import mate.academy.mapstruct.config.MapperConfig;
 import mate.academy.mapstruct.dto.subject.CreateSubjectRequestDto;
 import mate.academy.mapstruct.dto.subject.SubjectDto;
 import mate.academy.mapstruct.model.Subject;
+import org.mapstruct.Mapper;
 
+@Mapper(config = MapperConfig.class)
 public interface SubjectMapper {
     SubjectDto toDto(Subject subject);