[ci skip] remove guide section on models in migrations

Both examples given are incorrect, and no one can seem to come up with
a realistic situation where this is an actual problem. For now, it seems like
the best thing to do is just scrap the section altogether.

guides/source/migrations.md

@@ -818,159 +818,6 @@ The `revert` method can be helpful when writing a new migration to undo
 previous migrations in whole or in part
 (see [Reverting Previous Migrations](#reverting-previous-migrations) above).
 
-Using Models in Your Migrations
--------------------------------
-
-When creating or updating data in a migration it is often tempting to use one
-of your models. After all, they exist to provide easy access to the underlying
-data. This can be done, but some caution should be observed.
-
-For example, problems occur when the model uses database columns which are (1)
-not currently in the database and (2) will be created by this or a subsequent
-migration.
-
-Consider this example, where Alice and Bob are working on the same code base
-which contains a `Product` model:
-
-Bob goes on vacation.
-
-Alice creates a migration for the `products` table which adds a new column and
-initializes it:
-
-```ruby
-# db/migrate/20100513121110_add_flag_to_product.rb
-
-class AddFlagToProduct < ActiveRecord::Migration
-  def change
-    add_column :products, :flag, :boolean
-    reversible do |dir|
-      dir.up { Product.update_all flag: false }
-    end
-  end
-end
-```
-
-She also adds a validation to the `Product` model for the new column:
-
-```ruby
-# app/models/product.rb
-
-class Product < ActiveRecord::Base
-  validates :flag, inclusion: { in: [true, false] }
-end
-```
-
-Alice adds a second migration which adds another column to the `products`
-table and initializes it:
-
-```ruby
-# db/migrate/20100515121110_add_fuzz_to_product.rb
-
-class AddFuzzToProduct < ActiveRecord::Migration
-  def change
-    add_column :products, :fuzz, :string
-    reversible do |dir|
-      dir.up { Product.update_all fuzz: 'fuzzy' }
-    end
-  end
-end
-```
-
-She also adds a validation to the `Product` model for the new column:
-
-```ruby
-# app/models/product.rb
-
-class Product < ActiveRecord::Base
-  validates :flag, inclusion: { in: [true, false] }
-  validates :fuzz, presence: true
-end
-```
-
-Both migrations work for Alice.
-
-Bob comes back from vacation and:
-
-*   Updates the source - which contains both migrations and the latest version
-    of the Product model.
-*   Runs outstanding migrations with `rake db:migrate`, which
-    includes the one that updates the `Product` model.
-
-The migration crashes because when the model attempts to save, it tries to
-validate the second added column, which is not in the database when the _first_
-migration runs:
-
-```
-rake aborted!
-An error has occurred, this and all later migrations canceled:
-
-undefined method `fuzz' for #<Product:0x000001049b14a0>
-```
-
-A fix for this is to create a local model within the migration. This keeps
-Rails from running the validations, so that the migrations run to completion.
-
-When using a local model, it's a good idea to call
-`Product.reset_column_information` to refresh the Active Record cache for the
-`Product` model prior to updating data in the database.
-
-If Alice had done this instead, there would have been no problem:
-
-```ruby
-# db/migrate/20100513121110_add_flag_to_product.rb
-
-class AddFlagToProduct < ActiveRecord::Migration
-  class Product < ActiveRecord::Base
-  end
-
-  def change
-    add_column :products, :flag, :boolean
-    Product.reset_column_information
-    reversible do |dir|
-      dir.up { Product.update_all flag: false }
-    end
-  end
-end
-```
-
-```ruby
-# db/migrate/20100515121110_add_fuzz_to_product.rb
-
-class AddFuzzToProduct < ActiveRecord::Migration
-  class Product < ActiveRecord::Base
-  end
-
-  def change
-    add_column :products, :fuzz, :string
-    Product.reset_column_information
-    reversible do |dir|
-      dir.up { Product.update_all fuzz: 'fuzzy' }
-    end
-  end
-end
-```
-
-There are other ways in which the above example could have gone badly.
-
-For example, imagine that Alice creates a migration that selectively
-updates the `description` field on certain products. She runs the
-migration, commits the code, and then begins working on the next feature,
-which is to add a new column `fuzz` to the products table.
-
-She creates two migrations for this new feature, one which adds the new
-column, and a second which selectively updates the `fuzz` column based on
-other product attributes.
-
-These migrations run just fine, but when Bob comes back from his vacation
-and calls `rake db:migrate` to run all the outstanding migrations, he gets a
-subtle bug: The descriptions have defaults, and the `fuzz` column is present,
-but `fuzz` is `nil` on all products.
-
-The solution is again to use `Product.reset_column_information` before
-referencing the Product model in a migration, ensuring the Active Record's
-knowledge of the table structure is current before manipulating data in those
-records.
-
 Schema Dumping and You
 ----------------------