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-rw-r--r--guides/source/upgrading_ruby_on_rails.md17
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diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
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+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -156,6 +156,23 @@ end
ActiveRecord::FixtureSet.context_class.send :include, FixtureFileHelpers
```
+### Mutator methods called on Relation
+
+`Relation` no longer has mutator methods like `#map!` and `#delete_if`. Convert
+to an `Array` by calling `#to_a` before using these methods.
+
+It intends to prevent odd bugs and confusion in code that call mutator
+methods directly on the `Relation`.
+
+```ruby
+# Instead of this
+Author.where(name: 'Hank Moody').compact!
+
+# Now you have to do this
+authors = Author.where(name: 'Hank Moody').to_a
+authors.compact!
+```
+
Upgrading from Rails 3.2 to Rails 4.0
-------------------------------------