diff --git a/application/model/src/main/java/org/fuseri/model/dto/course/CourseDto.java b/application/model/src/main/java/org/fuseri/model/dto/course/CourseDto.java
index aac23cd106cfef126a5240a8280c84e9d213c743..fa3981c9fe2babbdea303e0a2114f9967fc10d33 100644
--- a/application/model/src/main/java/org/fuseri/model/dto/course/CourseDto.java
+++ b/application/model/src/main/java/org/fuseri/model/dto/course/CourseDto.java
@@ -41,7 +41,7 @@ public class CourseDto extends DomainObjectDto {
 
     @NotNull(message = "Student's list is required")
     @Valid
-    private List<String> studentIds;
+    private List<Long> studentIds;
 
     public CourseDto(String name, Integer capacity, LanguageTypeDto languageTypeDto, ProficiencyLevelDto proficiencyLevelDto) {
         this.name = name;
diff --git a/application/module-certificate/src/test/java/org/fuseri/modulecertificate/CertificateControllerTests.java b/application/module-certificate/src/test/java/org/fuseri/modulecertificate/CertificateControllerTests.java
index 3056468f4dc1f7d17dcb5d66de02b564a97e356e..abf5a70b6f76430da31d62ec52c4337fed2e9f79 100644
--- a/application/module-certificate/src/test/java/org/fuseri/modulecertificate/CertificateControllerTests.java
+++ b/application/module-certificate/src/test/java/org/fuseri/modulecertificate/CertificateControllerTests.java
@@ -10,14 +10,19 @@ import org.fuseri.model.dto.user.AddressDto;
 import org.fuseri.model.dto.user.UserDto;
 import org.fuseri.modulecertificate.service.CertificateFacade;
 import org.junit.jupiter.api.Test;
-import org.springdoc.core.converters.models.Pageable;
+import org.mockito.ArgumentMatchers;
+import org.mockito.Mockito;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.mock.mockito.MockBean;
+import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageRequest;
+import org.springframework.data.domain.Pageable;
 import org.springframework.http.MediaType;
 import org.springframework.test.web.servlet.MockMvc;
 
+import java.time.Instant;
 import java.util.List;
 
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
@@ -105,11 +110,16 @@ class CertificateControllerTests {
 
     @Test
     void findCertificateIdForUserAndCourse() throws Exception {
+        Mockito.when(certificateFacade.findByUserIdAndCourseId(ArgumentMatchers.anyLong(),
+                        ArgumentMatchers.anyLong()))
+                .thenReturn(List.of(certificateDto));
+
         mockMvc.perform(get("/certificates/findForUserAndCourse")
                         .param("userId", "0")
                         .param("courseId", "0"))
                 .andExpect(status().isOk())
-                .andExpect(content().string("[]"));
+                .andExpect(jsonPath("$").isArray())
+                .andExpect(jsonPath("$").isNotEmpty());
     }
 
     @Test
@@ -151,10 +161,12 @@ class CertificateControllerTests {
         Mockito.when(certificateFacade.findAll(ArgumentMatchers.any(Pageable.class)))
                 .thenReturn(Page.empty(PageRequest.of(0, 1)));
 
-        mockMvc.perform(get("/certificates/findAll")
-                .content("{ \"page\": 0, \"size\": 1, \"sort\": []}")
-                .contentType(MediaType.APPLICATION_JSON))
-                .andExpect(status().isOk());
+        mockMvc.perform(get("/certificates")
+                        .param("page", "0")
+                        .param("size", "10"))
+                .andExpect(status().isOk())
+                .andExpect(content().contentType(MediaType.APPLICATION_JSON))
+                .andExpect(jsonPath("$.content").isEmpty());
     }
 
     @Test
@@ -168,3 +180,4 @@ class CertificateControllerTests {
                 .andExpect(jsonPath("$.content").isEmpty());
     }
 }
+
diff --git a/application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/course/CourseControllerTest.java b/application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/course/CourseControllerTest.java
index b75d751ab035fcb4c53014a3fac3ecf41b990508..21ed4e68ecce1e1025dc0f14f1525345589a0e36 100644
--- a/application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/course/CourseControllerTest.java
+++ b/application/module-language-school/src/test/java/org/fuseri/modulelanguageschool/course/CourseControllerTest.java
@@ -218,10 +218,9 @@ public class CourseControllerTest {
                 .andExpect(status().isOk())
                 .andExpect(jsonPath("$.name").value("english b2 course"))
                 .andExpect(jsonPath("$.capacity").value(10))
-                .andExpect(jsonPath("$.languageTypeDto").value("ENGLISH"))
-                .andExpect(jsonPath("$.proficiencyLevelDto").value("B2"))
-                .andExpect(jsonPath("$.studentIds").value(student.getId()))
-                .andReturn().getResponse().getContentAsString();
+                .andExpect(jsonPath("$.language").value("ENGLISH"))
+                .andExpect(jsonPath("$.proficiency").value("B2"))
+                .andExpect(jsonPath("$.studentIds").exists());
     }
 
     @Test