Add Show Reconstructed Constraints For A Table as a Postgres TIL

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).
 
-_1625 TILs and counting..._
+_1626 TILs and counting..._
 
 See some of the other learning resources I work on:
 - [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -912,6 +912,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Sets With The Values Command](postgres/sets-with-the-values-command.md)
 - [Shorthand Absolute Value Operator](postgres/shorthand-absolute-value-operator.md)
 - [Show All Versions Of An Operator](postgres/show-all-versions-of-an-operator.md)
+- [Show Reconstructed Constraints For A Table](postgres/show-reconstructed-constraints-for-a-table.md)
 - [Show The Hidden Queries Behind Backslash Commands](postgres/show-the-hidden-queries-behind-backslash-commands.md)
 - [Sleeping](postgres/sleeping.md)
 - [Special Math Operators](postgres/special-math-operators.md)

postgres/show-reconstructed-constraints-for-a-table.md

@@ -0,0 +1,35 @@
+# Show Reconstructed Constraints For A Table
+
+The [`pg_get_constraintdef`
+function](https://pgpedia.info/p/pg_get_constraintdef.html) can be used to
+reconstruct the command for creating a given constraint. This isn't necessarily
+the command (or commands) that originally created the constraint, but rather a
+reconstruction.
+
+We have to pass it an `oid` that corresponds to the constraint which we can get
+from the `pg_constraint` table. These results can be further narrowed down by
+the `conname` (constraint name) and `conrelid` (table name).
+
+Here is an example of listing the constraints on a `reading_statuses` table.
+
+```sql
+> select
+    conname,
+    pg_get_constraintdef(oid)
+  from pg_constraint
+  where conrelid = 'reading_statuses'::regclass;
+
+               conname               |                                                                               pg_get_constraintdef
+-------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ reading_statuses_pkey               | PRIMARY KEY (id)
+ fk_rails_17ee7cb2c4                 | FOREIGN KEY (user_id) REFERENCES users(id)
+ fk_rails_0d3729339f                 | FOREIGN KEY (book_id) REFERENCES books(id)
+ reading_statuses_valid_status_check | CHECK (((status)::text = ANY ((ARRAY['started'::character varying, 'completed'::character varying, 'abandoned'::character varying, 'already_read'::character varying])::text[])))
+(4 rows)
+```
+
+I came across this while experimenting with [an idea for a fail-fast Rails
+initializer
+check](https://gist.github.com/jbranchaud/12813a0558f9cd06bcc24b7d8706550c)
+that verifies the values of the `reading_statuses_valid_status_check` stay in
+sync with the Rails version of those values that live in a constant.