Add Ensure Migrations Use The Latest Schema as a rails til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_812 TILs and counting..._
+_813 TILs and counting..._
 
 ---
 
@@ -493,6 +493,7 @@ _812 TILs and counting..._
 - [Delete Paranoid Records](rails/delete-paranoid-records.md)
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
+- [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Generating And Executing SQL](rails/generating-and-executing-sql.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)

rails/ensure-migrations-use-the-latest-schema.md

@@ -0,0 +1,50 @@
+# Ensure Migrations Use The Latest Schema
+
+Real-world migrations in Rails apps can sometimes involve both schema changes
+and data changes. For instance, if you are moving a column from one table to
+another, you'll need to add a new column, move some data, and then delete the
+old column.
+
+```ruby
+// Assume the following are defined:
+// GenericAuthor for table 'authors'
+// GenericBook for table 'books'
+
+def up
+  add_column :books, :genre, :string
+
+  GenericAuthor.find_each do |author|
+    book = GenericBook.find_by(author_id: author.id)
+    book.update!(genre: author.genre)
+  end
+
+  remove_column :authors, :genre
+end
+```
+
+This migration looks straightforward, but you may find that no data actually
+gets transferred to the `genre` column on `books`. This is because as a
+performance optimization, Rails has cached the scema. Thus an `ActiveRecord`
+modification like `book.update!` will be working off a version of the schema
+that doesn't include `genre` as a column.
+
+We can ensure `ActiveRecord` is using the latest column informtion for the
+`books` table by calling
+[`reset_column_information`](https://api.rubyonrails.org/classes/ActiveRecord/ModelSchema/ClassMethods.html#method-i-reset_column_information).
+
+```ruby
+def up
+  add_column :books, :genre, :string
+
+  GenericBook.reset_column_information
+
+  GenericAuthor.find_each do |author|
+    book = GenericBook.find_by(author_id: author.id)
+    book.update!(genre: author.genre)
+  end
+
+  remove_column :authors, :genre
+end
+```
+
+Now the update will work and `genre` will be set on `books`.