Global refactoring of the project to use poetry and implement tests,

fixing bugs, changing the handling of dto and db models, preparing to
add new functionality

library_service/routers/relationships.py

@@ -0,0 +1,50 @@
+from fastapi import APIRouter, Depends, HTTPException
+from sqlmodel import Session, select
+from typing import List, Dict
+
+from library_service.settings import get_session
+from library_service.models.db import Book, Author, AuthorBookLink
+
+router = APIRouter(prefix="/relationships", tags=["relations"])
+
+# Add author to book
+@router.post("/", response_model=AuthorBookLink)
+def add_author_to_book(author_id: int, book_id: int, session: Session = Depends(get_session)):
+    author = session.get(Author, author_id)
+    if not author:
+        raise HTTPException(status_code=404, detail="Author not found")
+
+    book = session.get(Book, book_id)
+    if not book:
+        raise HTTPException(status_code=404, detail="Book not found")
+
+    existing_link = session.exec(
+        select(AuthorBookLink)
+        .where(AuthorBookLink.author_id == author_id)
+        .where(AuthorBookLink.book_id == book_id)
+    ).first()
+
+    if existing_link:
+        raise HTTPException(status_code=400, detail="Relationship already exists")
+
+    link = AuthorBookLink(author_id=author_id, book_id=book_id)
+    session.add(link)
+    session.commit()
+    session.refresh(link)
+    return link
+
+# Remove author from book
+@router.delete("/", response_model=Dict[str, str])
+def remove_author_from_book(author_id: int, book_id: int, session: Session = Depends(get_session)):
+    link = session.exec(
+        select(AuthorBookLink)
+        .where(AuthorBookLink.author_id == author_id)
+        .where(AuthorBookLink.book_id == book_id)
+    ).first()
+
+    if not link:
+        raise HTTPException(status_code=404, detail="Relationship not found")
+
+    session.delete(link)
+    session.commit()
+    return {"message": "Relationship removed successfully"}