Add Drizzle Tracks Migrations In A Log Table as a Drizzle TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1489 TILs and counting..._
+_1490 TILs and counting..._
 
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@@ -212,6 +212,7 @@ _1489 TILs and counting..._
 ### Drizzle
 
 - [Create bigint Identity Column For Primary Key](drizzle/create-bigint-identity-column-for-primary-key.md)
+- [Drizzle Tracks Migrations In A Log Table](drizzle/drizzle-tracks-migrations-in-a-log-table.md)
 - [Get Fields For Inserted Row](drizzle/get-fields-for-inserted-row.md)
 
 ### Elixir

drizzle/drizzle-tracks-migrations-in-a-log-table.md

@@ -0,0 +1,39 @@
+# Drizzle Tracks Migrations In A Log Table
+
+When I generate (`npx drizzle-kit generate`) and apply (`npx drizzle-kit
+migrate`) schema migrations against my database with Drizzle, there are SQL
+files that get created and run.
+
+How does Drizzle know which SQL files have been run and which haven't?
+
+Like many SQL schema migration tools, it uses a table in the database to record
+this metadata. Drizzle defaults to calling this table `__drizzle_migrations`
+and puts it in the `drizzle` schema (which is like a database namespace).
+
+Let's take a look at this table for a project with two migrations:
+
+```sql
+postgres> \d drizzle.__drizzle_migrations
+                                  Table "drizzle.__drizzle_migrations"
+   Column   |  Type   | Collation | Nullable |                         Default
+------------+---------+-----------+----------+----------------------------------------------------------
+ id         | integer |           | not null | nextval('drizzle.__drizzle_migrations_id_seq'::regclass)
+ hash       | text    |           | not null |
+ created_at | bigint  |           |          |
+Indexes:
+    "__drizzle_migrations_pkey" PRIMARY KEY, btree (id)
+
+postgres> select * from drizzle.__drizzle_migrations;
+ id |                               hash                               |  created_at
+----+------------------------------------------------------------------+---------------
+  1 | 8961353bf66f9b3fe1a715f6ea9d9ef2bc65697bb8a5c2569df939a61e72a318 | 1730219291288
+  2 | b75e61451e2ce37d831608b1bc9231bf3af09e0ab54bf169be117de9d4ff6805 | 1730224013018
+(2 rows)
+```
+
+Notice that Drizzle stores each migration record as [a SHA256 hash of the
+migration
+file](https://github.com/drizzle-team/drizzle-orm/blob/526996bd2ea20d5b1a0d65e743b47e23329d441c/drizzle-orm/src/migrator.ts#L52)
+and a timestamp of when the migration was run.
+
+[source](https://orm.drizzle.team/docs/drizzle-kit-migrate#applied-migrations-log-in-the-database)