WEBVTT 00:00:00.350 --> 00:00:01.331 Welcome back! 00:00:01.511 --> 00:00:02.685 Here's what we'll learn. 00:00:02.865 --> 00:00:05.553 In lesson nine we supplied totals ourselves. 00:00:05.733 --> 00:00:09.231 Now Python hooks will calculate and validate them when we write. 00:00:09.411 --> 00:00:13.806 We will also see which direct SQL operations bypass this behavior. 00:00:13.986 --> 00:00:18.743 These are Python hooks in Asqueel, not SQL triggers installed in SQLite. 00:00:18.923 --> 00:00:20.737 We keep our database in memory. 00:00:22.150 --> 00:00:23.921 Let's define the write behavior. 00:00:24.101 --> 00:00:28.887 Our shared calculation checks quantity and price, then stores their product rounded to 00:00:28.887 --> 00:00:29.221 cents. 00:00:29.401 --> 00:00:32.857 The inserting and updating hooks run before the SQL write. 00:00:33.037 --> 00:00:36.557 The updated hook prints the old and new quantities afterwards. 00:00:36.737 --> 00:00:39.894 Afterwards means after the statement, not after commit. 00:00:41.224 --> 00:00:43.016 Let's attach it to invoice rows. 00:00:43.196 --> 00:00:47.207 X table class gives this existing table its Python behavior. 00:00:47.387 --> 00:00:50.032 Reload the model and get a fresh table handle. 00:00:50.212 --> 00:00:54.756 True means no migration is needed: we added Python hooks, not columns. 00:00:54.936 --> 00:00:57.325 Existing rows are not recalculated. 00:00:58.750 --> 00:01:00.179 Let's insert two teas. 00:01:00.359 --> 00:01:02.898 We supply quantity and price, but no total. 00:01:03.078 --> 00:01:04.785 The inserting hook stores seven. 00:01:04.965 --> 00:01:08.271 The formula column from lesson four also reads seven. 00:01:08.451 --> 00:01:13.145 The stored total is written by our hook; the formula is calculated when we query. 00:01:13.325 --> 00:01:15.053 This change is still uncommitted. 00:01:16.383 --> 00:01:18.153 Let's change only the quantity. 00:01:18.333 --> 00:01:23.624 Query update supplies the complete record and its previous values, as in lesson nine. 00:01:23.804 --> 00:01:27.708 This time our updating hook recalculates ten fifty automatically. 00:01:27.888 --> 00:01:30.576 The updated hook prints two to three before commit. 00:01:31.950 --> 00:01:34.254 Let's compare a direct SQL update. 00:01:34.434 --> 00:01:38.295 SQL update changes one selected row and returns one. 00:01:38.475 --> 00:01:43.531 It skips our Python hooks: no message appears, and the stored total stays ten fifty. 00:01:43.711 --> 00:01:45.589 The formula now reads fourteen. 00:01:45.769 --> 00:01:47.731 Database constraints still apply. 00:01:47.911 --> 00:01:53.394 The direct SQL insert used for CSV imports also skips these Python hooks. 00:01:54.724 --> 00:01:56.921 Let's run the normal write cycle again. 00:01:57.101 --> 00:01:59.597 Normal query update runs our hooks again. 00:01:59.777 --> 00:02:05.089 Both totals become seventeen fifty, and the result contains the updated primary key. 00:02:06.450 --> 00:02:08.477 Let's undo our demonstration writes. 00:02:08.657 --> 00:02:14.134 Rollback removes our new row and its updates together, returning us to thirty-six original 00:02:14.134 --> 00:02:14.438 rows. 00:02:14.618 --> 00:02:18.159 It does not erase messages already printed by a Python hook. 00:02:18.339 --> 00:02:23.395 Commit here closes the verification read; it does not save the rolled-back examples. 00:02:24.783 --> 00:02:26.746 Let's reject a zero quantity. 00:02:26.926 --> 00:02:31.001 Our inserting hook raises a value error before SQL insertion. 00:02:31.181 --> 00:02:34.637 After a failed write cycle, roll back before continuing. 00:02:34.817 --> 00:02:39.169 That rollback affects all pending work in the transaction, not just this row. 00:02:39.349 --> 00:02:42.826 We began from a clean boundary, so no earlier work is lost. 00:02:43.006 --> 00:02:47.379 The empty result confirms that row nine hundred and three was never stored. 00:02:48.783 --> 00:02:50.277 Thanks for following along! 00:02:50.457 --> 00:02:54.147 Use normal writes for Python validation and calculations. 00:02:54.327 --> 00:02:57.975 Use direct SQL deliberately, with complete valid data. 00:02:58.155 --> 00:03:00.289 Keep transaction boundaries explicit. 00:03:00.469 --> 00:03:04.906 We have added automatic write behavior to the operations from lesson nine.