Add Add A Database Index If It Does Not Already Exist as a Rails 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).
 
-_1348 TILs and counting..._
+_1349 TILs and counting..._
 
 ---
 
@@ -775,6 +775,7 @@ _1348 TILs and counting..._
 ### Rails
 
 - [Add A Check Constraint To A Table](rails/add-a-check-constraint-to-a-table.md)
+- [Add A Database Index If It Does Not Already Exist](rails/add-a-database-index-if-it-does-not-already-exist.md)
 - [Add A Foreign Key Reference To A Table](rails/add-a-foreign-key-reference-to-a-table.md)
 - [Add A Reference Column With An Index](rails/add-a-reference-column-with-an-index.md)
 - [Add ActiveRecord Error Not Tied To Any Attribute](rails/add-activerecord-error-not-tied-to-any-attribute.md)

rails/add-a-database-index-if-it-does-not-already-exist.md

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+# Add A Database Index If It Does Not Already Exist
+
+Sometimes you aren't sure if an index might already exist in one of the
+environments where a migration is going to run. But you still need to add the
+index elsewhere. One way of handling that is to add an index with the
+`if_not_exists` directive.
+
+```ruby
+class AddIndexToEventsCreatedAt < ActiveRecord::Migration[6.1]
+  def change
+    add_index :events, :created_at, if_not_exists: true
+  end
+end
+```
+
+`ActiveRecord` will translate this directive into the resulting SQL statement
+like so:
+
+```sql
+create index if not exists index_events_on_created_at on events ... ;
+```
+
+This way the index will be created in a database where it doesn't already exist
+and otherwise the statement will short-circuit rather than erroring when one
+does exist.
+
+A couple notes:
+
+1) From the PostgreSQL manual:
+
+> there is no guarantee that the existing index is anything like the one that
+> would have been created.
+
+2) The `if_not_exists` option also works with `create_table`.
+
+[source](https://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html)