diff --git a/tests/conftest.py b/tests/conftest.py
index 7dbf9c74..d170d5bb 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,4 +1,23 @@
 # tests/conftest.py - Updated main conftest with core fixtures only
+"""
+Core pytest configuration and fixtures.
+
+IMPORTANT - Fixture Best Practices:
+===================================
+1. DO NOT use db.expunge() on fixtures - it detaches objects from the session
+   and breaks lazy loading of relationships (e.g., product.marketplace_product).
+
+2. Test isolation is achieved through the db fixture which drops and recreates
+   all tables after each test - no need to manually detach objects.
+
+3. If you need to ensure an object has fresh data, use db.refresh(obj) instead
+   of expunge/re-query patterns.
+
+4. The session uses expire_on_commit=False to prevent objects from being expired
+   after commits, which helps with relationship access across operations.
+
+See docs/testing/testing-guide.md for comprehensive testing documentation.
+"""
 import pytest
 from fastapi.testclient import TestClient
 from sqlalchemy import create_engine
@@ -16,7 +35,7 @@ SQLALCHEMY_TEST_DATABASE_URL = "sqlite:///:memory:"
 
 @pytest.fixture(scope="session")
 def engine():
-    """Create test database engine"""
+    """Create test database engine."""
     return create_engine(
         SQLALCHEMY_TEST_DATABASE_URL,
         connect_args={"check_same_thread": False},
@@ -27,13 +46,34 @@ def engine():
 
 @pytest.fixture(scope="session")
 def testing_session_local(engine):
-    """Create session factory for tests"""
-    return sessionmaker(autocommit=False, autoflush=False, bind=engine)
+    """
+    Create session factory for tests.
+
+    Uses expire_on_commit=False to prevent objects from being expired after
+    commits. This allows fixtures to remain usable after database operations
+    without needing to refresh or re-query them.
+    """
+    return sessionmaker(
+        autocommit=False,
+        autoflush=False,
+        bind=engine,
+        expire_on_commit=False,  # Prevents lazy-load issues after commits
+    )
 
 
 @pytest.fixture(scope="function")
 def db(engine, testing_session_local):
-    """Create a database session for direct database operations"""
+    """
+    Create a database session for each test function.
+
+    Provides test isolation by:
+    - Creating fresh tables before each test
+    - Dropping all tables after each test completes
+
+    Note: Fixtures should NOT use db.expunge() as this detaches objects
+    from the session and breaks lazy loading. The table drop/create cycle
+    provides sufficient isolation between tests.
+    """
     # Create all tables
     Base.metadata.create_all(bind=engine)
 
diff --git a/tests/fixtures/auth_fixtures.py b/tests/fixtures/auth_fixtures.py
index c2618c66..8cb1a07e 100644
--- a/tests/fixtures/auth_fixtures.py
+++ b/tests/fixtures/auth_fixtures.py
@@ -1,4 +1,10 @@
 # tests/fixtures/auth_fixtures.py
+"""
+Authentication-related test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import uuid
 
 import pytest
@@ -9,14 +15,14 @@ from models.database.user import User
 
 @pytest.fixture(scope="session")
 def auth_manager():
-    """Create auth manager instance (session scope since it's stateless)"""
+    """Create auth manager instance (session scope since it's stateless)."""
     return AuthManager()
 
 
 @pytest.fixture
 def test_user(db, auth_manager):
-    """Create a test user with unique username"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create a test user with unique username."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("testpass123")
     user = User(
         email=f"test_{unique_id}@example.com",
@@ -28,16 +34,13 @@ def test_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    # Expunge user from session to prevent ResourceWarning about unclosed connections
-    # This detaches the object from the session so it doesn't hold a reference
-    db.expunge(user)
     return user
 
 
 @pytest.fixture
 def test_admin(db, auth_manager):
-    """Create a test admin user with unique username"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create a test admin user with unique username."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("adminpass123")
     admin = User(
         email=f"admin_{unique_id}@example.com",
@@ -49,15 +52,13 @@ def test_admin(db, auth_manager):
     db.add(admin)
     db.commit()
     db.refresh(admin)
-    # Expunge admin from session to prevent ResourceWarning about unclosed connections
-    db.expunge(admin)
     return admin
 
 
 @pytest.fixture
 def another_admin(db, auth_manager):
-    """Create another test admin user for testing admin-to-admin interactions"""
-    unique_id = str(uuid.uuid4())[:8]  # Short unique identifier
+    """Create another test admin user for testing admin-to-admin interactions."""
+    unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("anotheradminpass123")
     admin = User(
         email=f"another_admin_{unique_id}@example.com",
@@ -69,14 +70,12 @@ def another_admin(db, auth_manager):
     db.add(admin)
     db.commit()
     db.refresh(admin)
-    # Expunge admin from session to prevent ResourceWarning about unclosed connections
-    db.expunge(admin)
     return admin
 
 
 @pytest.fixture
 def other_user(db, auth_manager):
-    """Create a different user for testing access controls"""
+    """Create a different user for testing access controls."""
     unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("otherpass123")
     user = User(
@@ -89,8 +88,6 @@ def other_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    # Expunge user from session to prevent ResourceWarning about unclosed connections
-    db.expunge(user)
     return user
 
 
@@ -120,7 +117,7 @@ def admin_headers(client, test_admin):
 
 @pytest.fixture
 def test_vendor_user(db, auth_manager):
-    """Create a test vendor user with unique username"""
+    """Create a test vendor user with unique username."""
     unique_id = str(uuid.uuid4())[:8]
     hashed_password = auth_manager.hash_password("vendorpass123")
     user = User(
@@ -133,7 +130,6 @@ def test_vendor_user(db, auth_manager):
     db.add(user)
     db.commit()
     db.refresh(user)
-    db.expunge(user)
     return user
 
 
diff --git a/tests/fixtures/customer_fixtures.py b/tests/fixtures/customer_fixtures.py
index 5666834b..968a0e5e 100644
--- a/tests/fixtures/customer_fixtures.py
+++ b/tests/fixtures/customer_fixtures.py
@@ -1,12 +1,19 @@
 # tests/fixtures/customer_fixtures.py
+"""
+Customer-related test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import pytest
 
 from models.database.customer import Customer, CustomerAddress
+from models.database.order import Order
 
 
 @pytest.fixture
 def test_customer(db, test_vendor):
-    """Create a test customer"""
+    """Create a test customer."""
     customer = Customer(
         vendor_id=test_vendor.id,
         email="testcustomer@example.com",
@@ -19,14 +26,12 @@ def test_customer(db, test_vendor):
     db.add(customer)
     db.commit()
     db.refresh(customer)
-    # Expunge customer from session to prevent ResourceWarning about unclosed connections
-    db.expunge(customer)
     return customer
 
 
 @pytest.fixture
 def test_customer_address(db, test_vendor, test_customer):
-    """Create a test customer address"""
+    """Create a test customer address."""
     address = CustomerAddress(
         vendor_id=test_vendor.id,
         customer_id=test_customer.id,
@@ -42,6 +47,24 @@ def test_customer_address(db, test_vendor, test_customer):
     db.add(address)
     db.commit()
     db.refresh(address)
-    # Expunge address from session to prevent ResourceWarning about unclosed connections
-    db.expunge(address)
     return address
+
+
+@pytest.fixture
+def test_order(db, test_vendor, test_customer, test_customer_address):
+    """Create a test order."""
+    order = Order(
+        vendor_id=test_vendor.id,
+        customer_id=test_customer.id,
+        order_number="TEST-ORD-001",
+        status="pending",
+        subtotal=99.99,
+        total_amount=99.99,
+        currency="EUR",
+        shipping_address_id=test_customer_address.id,
+        billing_address_id=test_customer_address.id,
+    )
+    db.add(order)
+    db.commit()
+    db.refresh(order)
+    return order
diff --git a/tests/fixtures/marketplace_import_job_fixtures.py b/tests/fixtures/marketplace_import_job_fixtures.py
index c79decd7..1915a474 100644
--- a/tests/fixtures/marketplace_import_job_fixtures.py
+++ b/tests/fixtures/marketplace_import_job_fixtures.py
@@ -1,4 +1,10 @@
 # tests/fixtures/marketplace_import_job_fixtures.py
+"""
+Marketplace import job test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import pytest
 
 from models.database.marketplace_import_job import MarketplaceImportJob
@@ -6,10 +12,9 @@ from models.database.marketplace_import_job import MarketplaceImportJob
 
 @pytest.fixture
 def test_marketplace_import_job(db, test_vendor, test_user):
-    """Create a test marketplace import job"""
+    """Create a test marketplace import job."""
     job = MarketplaceImportJob(
         marketplace="amazon",
-        # REMOVED: vendor_name field doesn't exist
         status="completed",
         source_url="https://test-marketplace.example.com/import",
         vendor_id=test_vendor.id,
@@ -23,16 +28,13 @@ def test_marketplace_import_job(db, test_vendor, test_user):
     db.add(job)
     db.commit()
     db.refresh(job)
-    # Expunge job from session to prevent ResourceWarning about unclosed connections
-    db.expunge(job)
     return job
 
 
 def create_test_marketplace_import_job(db, vendor_id, user_id, **kwargs):
-    """Helper function to create MarketplaceImportJob with defaults"""
+    """Helper function to create MarketplaceImportJob with defaults."""
     defaults = {
         "marketplace": "test",
-        # REMOVED: name field
         "status": "pending",
         "source_url": "https://test.example.com/import",
         "vendor_id": vendor_id,
@@ -49,6 +51,4 @@ def create_test_marketplace_import_job(db, vendor_id, user_id, **kwargs):
     db.add(job)
     db.commit()
     db.refresh(job)
-    # Expunge job from session to prevent ResourceWarning about unclosed connections
-    db.expunge(job)
     return job
diff --git a/tests/fixtures/marketplace_product_fixtures.py b/tests/fixtures/marketplace_product_fixtures.py
index a27f9b87..e83d0fd3 100644
--- a/tests/fixtures/marketplace_product_fixtures.py
+++ b/tests/fixtures/marketplace_product_fixtures.py
@@ -1,4 +1,10 @@
 # tests/fixtures/marketplace_product_fixtures.py
+"""
+Marketplace product test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import uuid
 
 import pytest
@@ -8,7 +14,7 @@ from models.database.marketplace_product import MarketplaceProduct
 
 @pytest.fixture
 def test_marketplace_product(db):
-    """Create a test product"""
+    """Create a test product."""
     marketplace_product = MarketplaceProduct(
         marketplace_product_id="TEST001",
         title="Test MarketplaceProduct",
@@ -24,14 +30,12 @@ def test_marketplace_product(db):
     db.add(marketplace_product)
     db.commit()
     db.refresh(marketplace_product)
-    # Expunge marketplace_product from session to prevent ResourceWarning about unclosed connections
-    db.expunge(marketplace_product)
     return marketplace_product
 
 
 @pytest.fixture
 def unique_product(db):
-    """Create a unique product for tests that need isolated product data"""
+    """Create a unique product for tests that need isolated product data."""
     unique_id = str(uuid.uuid4())[:8]
     marketplace_product = MarketplaceProduct(
         marketplace_product_id=f"UNIQUE_{unique_id}",
@@ -49,14 +53,12 @@ def unique_product(db):
     db.add(marketplace_product)
     db.commit()
     db.refresh(marketplace_product)
-    # Expunge marketplace_product from session to prevent ResourceWarning about unclosed connections
-    db.expunge(marketplace_product)
     return marketplace_product
 
 
 @pytest.fixture
 def multiple_products(db):
-    """Create multiple products for testing statistics and pagination"""
+    """Create multiple products for testing statistics and pagination."""
     unique_id = str(uuid.uuid4())[:8]
     marketplace_products = []
 
@@ -79,13 +81,11 @@ def multiple_products(db):
     db.commit()
     for product in marketplace_products:
         db.refresh(product)
-        # Expunge each product from session to prevent ResourceWarning about unclosed connections
-        db.expunge(product)
     return marketplace_products
 
 
 def create_unique_marketplace_product_factory():
-    """Factory function to create unique products in tests"""
+    """Factory function to create unique products in tests."""
 
     def _marketplace_create_product(db, **kwargs):
         unique_id = str(uuid.uuid4())[:8]
@@ -103,8 +103,6 @@ def create_unique_marketplace_product_factory():
         db.add(marketplace_product)
         db.commit()
         db.refresh(marketplace_product)
-        # Expunge marketplace_product from session to prevent ResourceWarning about unclosed connections
-        db.expunge(marketplace_product)
         return marketplace_product
 
     return _marketplace_create_product
diff --git a/tests/fixtures/vendor_fixtures.py b/tests/fixtures/vendor_fixtures.py
index 4b2443e4..c841556e 100644
--- a/tests/fixtures/vendor_fixtures.py
+++ b/tests/fixtures/vendor_fixtures.py
@@ -1,44 +1,96 @@
 # tests/fixtures/vendor_fixtures.py
+"""
+Vendor-related test fixtures.
+
+Note: Fixtures should NOT use db.expunge() as it breaks lazy loading.
+See tests/conftest.py for details on fixture best practices.
+"""
 import uuid
 
 import pytest
 
+from models.database.company import Company
 from models.database.inventory import Inventory
 from models.database.product import Product
 from models.database.vendor import Vendor
 
 
 @pytest.fixture
-def test_vendor(db, test_user):
-    """Create a test vendor with unique vendor code"""
+def test_company(db, test_user):
+    """Create a test company owned by test_user."""
+    unique_id = str(uuid.uuid4())[:8]
+    company = Company(
+        name=f"Test Company {unique_id}",
+        owner_user_id=test_user.id,
+        contact_email=f"test{unique_id}@company.com",
+        is_active=True,
+        is_verified=True,
+    )
+    db.add(company)
+    db.commit()
+    db.refresh(company)
+    return company
+
+
+@pytest.fixture
+def other_company(db, other_user):
+    """Create a test company owned by other_user."""
+    unique_id = str(uuid.uuid4())[:8]
+    company = Company(
+        name=f"Other Company {unique_id}",
+        owner_user_id=other_user.id,
+        contact_email=f"other{unique_id}@company.com",
+        is_active=True,
+        is_verified=True,
+    )
+    db.add(company)
+    db.commit()
+    db.refresh(company)
+    return company
+
+
+@pytest.fixture
+def test_vendor(db, test_company):
+    """Create a test vendor with unique vendor code."""
     unique_id = str(uuid.uuid4())[:8].upper()
     vendor = Vendor(
+        company_id=test_company.id,
         vendor_code=f"TESTVENDOR_{unique_id}",
-        subdomain=f"testvendor{unique_id.lower()}",  # ADDED
-        name=f"Test Vendor {unique_id.lower()}",  # FIXED
-        owner_user_id=test_user.id,
+        subdomain=f"testvendor{unique_id.lower()}",
+        name=f"Test Vendor {unique_id.lower()}",
         is_active=True,
         is_verified=True,
     )
     db.add(vendor)
     db.commit()
     db.refresh(vendor)
-    # Expunge vendor from session to prevent ResourceWarning about unclosed connections
-    db.expunge(vendor)
     return vendor
 
 
 @pytest.fixture
 def test_vendor_with_vendor_user(db, test_vendor_user):
-    """Create a vendor owned by a vendor user (for testing vendor API endpoints)"""
-    from models.database.vendor import VendorUser
+    """Create a vendor owned by a vendor user (for testing vendor API endpoints)."""
+    from models.database.vendor import VendorUser, VendorUserType
 
     unique_id = str(uuid.uuid4())[:8].upper()
+
+    # Create company first
+    company = Company(
+        name=f"Vendor API Company {unique_id}",
+        owner_user_id=test_vendor_user.id,
+        contact_email=f"vendorapi{unique_id}@company.com",
+        is_active=True,
+        is_verified=True,
+    )
+    db.add(company)
+    db.commit()
+    db.refresh(company)
+
     vendor = Vendor(
+        company_id=company.id,
         vendor_code=f"VENDORAPI_{unique_id}",
         subdomain=f"vendorapi{unique_id.lower()}",
         name=f"Vendor API Test {unique_id}",
-        owner_user_id=test_vendor_user.id,
         is_active=True,
         is_verified=True,
     )
@@ -50,81 +102,73 @@ def test_vendor_with_vendor_user(db, test_vendor_user):
     vendor_user = VendorUser(
         vendor_id=vendor.id,
         user_id=test_vendor_user.id,
-        is_owner=True,
+        user_type=VendorUserType.OWNER.value,
         is_active=True,
     )
     db.add(vendor_user)
     db.commit()
     db.refresh(vendor)
-
-    db.expunge(vendor)
     return vendor
 
 
 @pytest.fixture
-def unique_vendor(db, test_user):
-    """Create a unique vendor for tests that need isolated vendor data"""
+def unique_vendor(db, test_company):
+    """Create a unique vendor for tests that need isolated vendor data."""
     unique_id = str(uuid.uuid4())[:8]
     vendor = Vendor(
+        company_id=test_company.id,
         vendor_code=f"UNIQUEVENDOR_{unique_id.upper()}",
-        subdomain=f"uniquevendor{unique_id.lower()}",  # ADDED
-        name=f"Unique Test Vendor {unique_id}",  # FIXED
+        subdomain=f"uniquevendor{unique_id.lower()}",
+        name=f"Unique Test Vendor {unique_id}",
         description=f"A unique test vendor {unique_id}",
-        owner_user_id=test_user.id,
         is_active=True,
         is_verified=True,
     )
     db.add(vendor)
     db.commit()
     db.refresh(vendor)
-    # Expunge vendor from session to prevent ResourceWarning about unclosed connections
-    db.expunge(vendor)
     return vendor
 
 
 @pytest.fixture
-def inactive_vendor(db, other_user):
-    """Create an inactive vendor owned by other_user"""
+def inactive_vendor(db, other_company):
+    """Create an inactive vendor owned by other_user."""
     unique_id = str(uuid.uuid4())[:8]
     vendor = Vendor(
+        company_id=other_company.id,
         vendor_code=f"INACTIVE_{unique_id.upper()}",
-        subdomain=f"inactive{unique_id.lower()}",  # ADDED
-        name=f"Inactive Vendor {unique_id}",  # FIXED
-        owner_user_id=other_user.id,
+        subdomain=f"inactive{unique_id.lower()}",
+        name=f"Inactive Vendor {unique_id}",
         is_active=False,
         is_verified=False,
     )
     db.add(vendor)
     db.commit()
     db.refresh(vendor)
-    # Expunge vendor from session to prevent ResourceWarning about unclosed connections
-    db.expunge(vendor)
     return vendor
 
 
 @pytest.fixture
-def verified_vendor(db, other_user):
-    """Create a verified vendor owned by other_user"""
+def verified_vendor(db, other_company):
+    """Create a verified vendor owned by other_user."""
     unique_id = str(uuid.uuid4())[:8]
     vendor = Vendor(
+        company_id=other_company.id,
         vendor_code=f"VERIFIED_{unique_id.upper()}",
-        subdomain=f"verified{unique_id.lower()}",  # ADDED
-        name=f"Verified Vendor {unique_id}",  # FIXED
-        owner_user_id=other_user.id,
+        subdomain=f"verified{unique_id.lower()}",
+        name=f"Verified Vendor {unique_id}",
         is_active=True,
         is_verified=True,
     )
     db.add(vendor)
     db.commit()
     db.refresh(vendor)
-    # Expunge vendor from session to prevent ResourceWarning about unclosed connections
-    db.expunge(vendor)
     return vendor
 
 
 @pytest.fixture
 def test_product(db, test_vendor, unique_product):
-    """Create a vendor product relationship"""
+    """Create a vendor product relationship."""
     product = Product(
         vendor_id=test_vendor.id,
         marketplace_product_id=unique_product.id,
@@ -136,8 +180,6 @@ def test_product(db, test_vendor, unique_product):
     db.add(product)
     db.commit()
     db.refresh(product)
-    # Expunge product from session to prevent ResourceWarning about unclosed connections
-    db.expunge(product)
     return product
 
 
@@ -151,19 +193,17 @@ def test_inventory(db, test_product):
         location=f"WAREHOUSE_A_{unique_id}",
         quantity=100,
         reserved_quantity=10,
-        gtin=test_product.marketplace_product.gtin,  # Optional reference
+        gtin=test_product.marketplace_product.gtin,
     )
     db.add(inventory)
     db.commit()
     db.refresh(inventory)
-    # Expunge inventory from session to prevent ResourceWarning about unclosed connections
-    db.expunge(inventory)
     return inventory
 
 
 @pytest.fixture
 def multiple_inventory_entries(db, multiple_products, test_vendor):
-    """Create multiple inventory entries for testing"""
+    """Create multiple inventory entries for testing."""
     inventory_entries = []
     for i, product in enumerate(multiple_products):
         inventory = Inventory(
@@ -179,21 +219,19 @@ def multiple_inventory_entries(db, multiple_products, test_vendor):
     db.commit()
     for inventory in inventory_entries:
         db.refresh(inventory)
-        # Expunge each inventory from session to prevent ResourceWarning about unclosed connections
-        db.expunge(inventory)
     return inventory_entries
 
 
 def create_unique_vendor_factory():
-    """Factory function to create unique vendors in tests"""
+    """Factory function to create unique vendors in tests."""
 
-    def _create_vendor(db, owner_user_id, **kwargs):
+    def _create_vendor(db, company_id, **kwargs):
         unique_id = str(uuid.uuid4())[:8]
         defaults = {
+            "company_id": company_id,
             "vendor_code": f"FACTORY_{unique_id.upper()}",
-            "subdomain": f"factory{unique_id.lower()}",  # ADDED
-            "name": f"Factory Vendor {unique_id}",  # FIXED
-            "owner_user_id": owner_user_id,
+            "subdomain": f"factory{unique_id.lower()}",
+            "name": f"Factory Vendor {unique_id}",
             "is_active": True,
             "is_verified": False,
         }
@@ -210,5 +248,5 @@ def create_unique_vendor_factory():
 
 @pytest.fixture
 def vendor_factory():
-    """Fixture that provides a vendor factory function"""
+    """Fixture that provides a vendor factory function."""
     return create_unique_vendor_factory()