Protect Both Checkout Rules

Foreign keys cannot express every lending decision. An item may already have an active loan, and a valid member may already be at the stored loan limit. One public checkout test should arrange both conflicts and require each distinct explanation.

Continue in tests/test_catalog.py. Complete test_checkout_refuses_active_items_and_full_members(tmp_path) using create_fresh_database(tmp_path, "checkout-rules.db") and catalog_db.open_database(database_path). Insert three items one at a time or in a small loop, plus two members. Give the first member room for another loan. Give the second member a limit of 1 and one active loan. Add a separate active loan for one of the first member’s items, then commit the arrangement.

For the active item, call only the public boundary:

catalog_db.checkout_item(connection, loan_id, item_id, member_id, checked_out_on)

Capture its ValueError with pytest.raises and compare the exact text That item is already checked out.. Next, attempt a different available item for the full member and require the exact text That member has reached the loan limit.. The member-limit wording includes “the loan limit”; keep that complete message.

Close the acting connection and reopen the same path. Select every loan field in ID order and compare the rows with the two committed loans you arranged. That complete comparison proves neither refusal inserted a row or changed an older one.

The test should not call a private validation function or repeat the decision with its own SELECT COUNT(*). Your arrangement establishes the conditions; checkout_item owns the decision. Testing any ValueError is also too broad because it would allow the two meanings to be swapped.

Press Run. The fixed entry invokes pytest.main(["-q", "-p", "no:cacheprovider", "tests/test_catalog.py"]) and should print:

Database tests passed: 4

Submit checks the active-item and member-limit behaviors separately. Removing either rule must make the suite fail while all four tests still collect. You are protecting two public meanings, not one general failure branch.