Use DidYouMean for AssociationNotFoundError

If an association isn't found we can suggest matching associations:

```
Post.all.merge!(includes: :monkeys).find(6)

Association named 'monkeys' was not found on Post; perhaps you misspelled it?
Did you mean?  funky_tags
               comments
               images
               skimmers
```

activerecord/lib/active_record/associations.rb

@@ -5,13 +5,39 @@ require "active_support/core_ext/string/conversions"
 
 module ActiveRecord
   class AssociationNotFoundError < ConfigurationError #:nodoc:
+    attr_reader :record, :association_name
     def initialize(record = nil, association_name = nil)
+      @record           = record
+      @association_name = association_name
       if record && association_name
         super("Association named '#{association_name}' was not found on #{record.class.name}; perhaps you misspelled it?")
       else
         super("Association was not found.")
       end
     end
+
+    class Correction
+      def initialize(error)
+        @error = error
+      end
+
+      def corrections
+        if @error.association_name
+          maybe_these = @error.record.class.reflections.keys
+
+          maybe_these.sort_by { |n|
+            DidYouMean::Jaro.distance(@error.association_name.to_s, n)
+          }.reverse.first(4)
+        else
+          []
+        end
+      end
+    end
+
+    # We may not have DYM, and DYM might not let us register error handlers
+    if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
+      DidYouMean.correct_error(self, Correction)
+    end
   end
 
   class InverseOfAssociationNotFoundError < ActiveRecordError #:nodoc:

activerecord/test/cases/associations/eager_test.rb

@@ -814,22 +814,31 @@ class EagerAssociationTest < ActiveRecord::TestCase
     e = assert_raise(ActiveRecord::AssociationNotFoundError) {
       Post.all.merge!(includes: :monkeys).find(6)
     }
-    assert_equal("Association named 'monkeys' was not found on Post; perhaps you misspelled it?", e.message)
+    assert_match(/Association named 'monkeys' was not found on Post; perhaps you misspelled it\?/, e.message)
 
     e = assert_raise(ActiveRecord::AssociationNotFoundError) {
       Post.all.merge!(includes: [ :monkeys ]).find(6)
     }
-    assert_equal("Association named 'monkeys' was not found on Post; perhaps you misspelled it?", e.message)
+    assert_match(/Association named 'monkeys' was not found on Post; perhaps you misspelled it\?/, e.message)
 
     e = assert_raise(ActiveRecord::AssociationNotFoundError) {
       Post.all.merge!(includes: [ "monkeys" ]).find(6)
     }
-    assert_equal("Association named 'monkeys' was not found on Post; perhaps you misspelled it?", e.message)
+    assert_match(/Association named 'monkeys' was not found on Post; perhaps you misspelled it\?/, e.message)
 
     e = assert_raise(ActiveRecord::AssociationNotFoundError) {
       Post.all.merge!(includes: [ :monkeys, :elephants ]).find(6)
     }
-    assert_equal("Association named 'monkeys' was not found on Post; perhaps you misspelled it?", e.message)
+    assert_match(/Association named 'monkeys' was not found on Post; perhaps you misspelled it\?/, e.message)
+  end
+
+  if defined?(DidYouMean) && DidYouMean.respond_to?(:correct_error)
+    test "exceptions have suggestions for fix" do
+      error = assert_raise(ActiveRecord::AssociationNotFoundError) {
+        Post.all.merge!(includes: :monkeys).find(6)
+      }
+      assert_match "Did you mean?", error.message
+    end
   end
 
   def test_eager_has_many_through_with_order