diff --git a/models.py b/models.py
index 8ee7611..9b34b72 100644
--- a/models.py
+++ b/models.py
@@ -3,7 +3,7 @@
 from pydantic import BaseModel, Field
 
 class Book(BaseModel):
-    id: str = Field(default_factory=uuid.uuid4, alias="_id")
+    id: str = Field()
     title: str = Field(...)
     author: str = Field(...)
     synopsis: str = Field(...)
@@ -12,13 +12,27 @@ class Config:
         allow_population_by_field_name = True
         schema_extra = {
             "example": {
-                "_id": "066de609-b04a-4b30-b46c-32537c7f1f6e",
+                "_id": "6428a5fffe4c6065ecf51cb7",
                 "title": "Don Quixote",
                 "author": "Miguel de Cervantes",
                 "synopsis": "..."
             }
         }
 
+class BookCreate(BaseModel):
+    title: str = Field(...)
+    author: str = Field(...)
+    synopsis: str = Field(...)
+
+    class Config:
+        allow_population_by_field_name = True
+        schema_extra = {
+            "example": {
+                "title": "Don Quixote",
+                "author": "Miguel de Cervantes",
+                "synopsis": "..."
+            }
+        }
 
 class BookUpdate(BaseModel):
     title: Optional[str]
diff --git a/requirements.txt b/requirements.txt
index df4ae42..f1ff0b6 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
-fastapi[all]==0.75.0
-pydantic==1.9
-pymongo[srv]==3.11.0
-python-dotenv==0.19.2
-pytest==7.0.1
+fastapi[all]==0.94.1
+pydantic==1.10.6
+pymongo[srv]==4.3.3
+python-dotenv==1.0.0
+pytest==7.2.2
\ No newline at end of file
diff --git a/routes.py b/routes.py
index f6f44e1..270efc5 100644
--- a/routes.py
+++ b/routes.py
@@ -1,31 +1,41 @@
 from fastapi import APIRouter, Body, Request, Response, HTTPException, status
 from fastapi.encoders import jsonable_encoder
 from typing import List
-
-from models import Book, BookUpdate
+from bson import ObjectId
+from models import Book, BookUpdate, BookCreate
 
 router = APIRouter()
 
-@router.post("/", response_description="Create a new book", status_code=status.HTTP_201_CREATED, response_model=Book)
-def create_book(request: Request, book: Book = Body(...)):
+@router.post("/", response_description="Create a new book", status_code=status.HTTP_201_CREATED)
+def create_book(request: Request, book: BookCreate = Body(...)):
     book = jsonable_encoder(book)
+
+    # verify if book title already exists
+    if request.app.database["books"].find_one({"title": book["title"]}):
+        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=f"Book with title {book['title']} already exists")
+
+
     new_book = request.app.database["books"].insert_one(book)
     created_book = request.app.database["books"].find_one(
         {"_id": new_book.inserted_id}
     )
 
-    return created_book
+    created_book['id'] = str(created_book['_id'])
+    return Book(**created_book)
 
 
 @router.get("/", response_description="List all books", response_model=List[Book])
 def list_books(request: Request):
     books = list(request.app.database["books"].find(limit=100))
+    for book in books:
+        book['id'] = str(book['_id'])
     return books
 
 
 @router.get("/{id}", response_description="Get a single book by id", response_model=Book)
 def find_book(id: str, request: Request):
-    if (book := request.app.database["books"].find_one({"_id": id})) is not None:
+    if (book := request.app.database["books"].find_one({"_id": ObjectId(id)})) is not None:
+        book['id'] = str(book['_id'])
         return book
 
     raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Book with ID {id} not found")
@@ -37,15 +47,16 @@ def update_book(id: str, request: Request, book: BookUpdate = Body(...)):
 
     if len(book) >= 1:
         update_result = request.app.database["books"].update_one(
-            {"_id": id}, {"$set": book}
+            {"_id": ObjectId(id)}, {"$set": book}
         )
 
         if update_result.modified_count == 0:
             raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Book with ID {id} not found")
 
     if (
-        existing_book := request.app.database["books"].find_one({"_id": id})
+        existing_book := request.app.database["books"].find_one({"_id": ObjectId(id)})
     ) is not None:
+        existing_book['id'] = str(existing_book['_id'])
         return existing_book
 
     raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Book with ID {id} not found")
@@ -53,7 +64,7 @@ def update_book(id: str, request: Request, book: BookUpdate = Body(...)):
 
 @router.delete("/{id}", response_description="Delete a book")
 def delete_book(id: str, request: Request, response: Response):
-    delete_result = request.app.database["books"].delete_one({"_id": id})
+    delete_result = request.app.database["books"].delete_one({"_id": ObjectId(id)})
 
     if delete_result.deleted_count == 1:
         response.status_code = status.HTTP_204_NO_CONTENT