diff --git a/src/models/__init__.py b/src/models/__init__.py
index dee725c..34e40bd 100644
--- a/src/models/__init__.py
+++ b/src/models/__init__.py
@@ -1,7 +1,10 @@
+from .base import Base
 from .author import Author
-from .book_category import BookCategory
 from .book import Book
+from .book_category import BookCategory
 from .book_category_link import BookCategoryLink
 from .member import Member
 from .librarian import Librarian
-from .loan import Loan
\ No newline at end of file
+from .loan import Loan
+
+__all__ = ["Author", "Book", "BookCategory", "BookCategoryLink", "Member", "Librarian", "Loan"]
diff --git a/src/models/author.py b/src/models/author.py
index 40e56f8..93fdd45 100644
--- a/src/models/author.py
+++ b/src/models/author.py
@@ -1,15 +1,17 @@
 from sqlalchemy import Column, Integer, String, TIMESTAMP, UniqueConstraint, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
 
-Base = declarative_base()
+from .base import Base
 
 
 class Author(Base):
-    __tablename__  = 'author'
+    __tablename__ = 'author'
     __table_args__ = (UniqueConstraint('id'),)
 
-    id           = Column(Integer, primary_key=True, autoincrement=True)
-    first_name   = Column(String(50), nullable=False)
-    last_name    = Column(String(50), nullable=False)
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    first_name = Column(String(50), nullable=False)
+    last_name = Column(String(50), nullable=False)
     last_updated = Column(TIMESTAMP, nullable=False, server_default=func.now())
+
+    # Reference 'Book' as a string to avoid direct import
+    books = relationship('Book', back_populates='author')
diff --git a/src/models/base.py b/src/models/base.py
new file mode 100644
index 0000000..860e542
--- /dev/null
+++ b/src/models/base.py
@@ -0,0 +1,3 @@
+from sqlalchemy.ext.declarative import declarative_base
+
+Base = declarative_base()
diff --git a/src/models/book.py b/src/models/book.py
index ce96965..6e86071 100644
--- a/src/models/book.py
+++ b/src/models/book.py
@@ -2,12 +2,8 @@ import enum
 
 from sqlalchemy import Column, Integer, String, TIMESTAMP, Text, ForeignKey, Enum, UniqueConstraint, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
 
-from .author import Author
-from .book_category import BookCategory
-
-Base = declarative_base()
+from .base import Base
 
 
 class BookStatusEnum(enum.Enum):
@@ -15,18 +11,20 @@ class BookStatusEnum(enum.Enum):
     borrowed = 'borrowed'
     reserved = 'reserved'
 
+
 class Book(Base):
-    __tablename__  = 'book'
+    __tablename__ = 'book'
     __table_args__ = (UniqueConstraint('isbn'),)
 
-    id             = Column(Integer, primary_key=True, autoincrement=True)
-    author_id      = Column(Integer, ForeignKey('author.id'), nullable=False)
-    title          = Column(String(100), nullable=False)
-    description    = Column(Text, nullable=False)
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    author_id = Column(Integer, ForeignKey('author.id'), nullable=False)
+    title = Column(String(100), nullable=False)
+    description = Column(Text, nullable=False)
     year_published = Column(String(4), nullable=False)
-    isbn           = Column(String(13), nullable=False, unique=True)
-    status         = Column(Enum(BookStatusEnum), nullable=False, default=BookStatusEnum.available)
-    created_at     = Column(TIMESTAMP, nullable=False, server_default=func.now())
-    last_updated   = Column(TIMESTAMP, nullable=False, server_default=func.now())
+    isbn = Column(String(13), nullable=False, unique=True)
+    status = Column(Enum(BookStatusEnum), nullable=False, default=BookStatusEnum.available)
+    created_at = Column(TIMESTAMP, nullable=False, server_default=func.now())
+    last_updated = Column(TIMESTAMP, nullable=False, server_default=func.now())
 
-    author = relationship('Author', backref='books')
+    # Reference 'Author' as a string to avoid direct import
+    author = relationship('Author', back_populates='books')
diff --git a/src/models/book_category.py b/src/models/book_category.py
index d0d99eb..eb901a6 100644
--- a/src/models/book_category.py
+++ b/src/models/book_category.py
@@ -1,9 +1,7 @@
 from sqlalchemy import Column, Integer, String, TIMESTAMP, ForeignKey, UniqueConstraint, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
-
-Base = declarative_base()
 
+from .base import Base
 
 class BookCategory(Base):
     __tablename__  = 'book_category'
diff --git a/src/models/book_category_link.py b/src/models/book_category_link.py
index adde605..0bc52ad 100644
--- a/src/models/book_category_link.py
+++ b/src/models/book_category_link.py
@@ -1,12 +1,9 @@
 from sqlalchemy import Column, Integer, ForeignKey
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
-
 
 from .book import Book
 from .book_category import BookCategory
-
-Base = declarative_base()
+from .base import Base
 
 class BookCategoryLink(Base):
     __tablename__ = 'book_category_link'
diff --git a/src/models/librarian.py b/src/models/librarian.py
index 7a49364..019e067 100644
--- a/src/models/librarian.py
+++ b/src/models/librarian.py
@@ -2,10 +2,8 @@ import enum
 
 from sqlalchemy import Column, Integer, String, TIMESTAMP, Text, ForeignKey, Enum, UniqueConstraint, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
 
-
-Base = declarative_base()
+from .base import Base
 
 
 class LibrarianStatusEnum(enum.Enum):
diff --git a/src/models/loan.py b/src/models/loan.py
index a82f922..2e34451 100644
--- a/src/models/loan.py
+++ b/src/models/loan.py
@@ -2,14 +2,12 @@ import enum
 
 from sqlalchemy import Column, Integer, String, TIMESTAMP, Text, ForeignKey, Enum, UniqueConstraint, Float, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
 
+from .base import Base
 from .book import Book
 from .member import Member
 from .librarian import Librarian
 
-Base = declarative_base()
-
 
 class LoanStatusEnum(enum.Enum):
     borrowed = 'borrowed'
diff --git a/src/models/member.py b/src/models/member.py
index 9d9c97f..79ee7dc 100644
--- a/src/models/member.py
+++ b/src/models/member.py
@@ -2,13 +2,14 @@ import enum
 
 from sqlalchemy import Column, Integer, String, TIMESTAMP, Text, ForeignKey, Enum, UniqueConstraint, func
 from sqlalchemy.orm import relationship
-from sqlalchemy.ext.declarative import declarative_base
+
+from .base import Base
+
 
 class MemberStatusEnum(enum.Enum):
     active = 'active'
     inactive = 'inactive'
 
-Base = declarative_base()
 
 class Member(Base):
     __tablename__  = 'member'