`index_exists?` with `:name` checks specified columns.

[Yves Senn & Matthew Draper]

The column check was embodied in the defaul index name.
If the :name option was used, the specified columns were not verified at all.

Given:

```
assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_yo_momma)
```

That index could have been defined on any field, not necessarily on `:foo_id`.

activerecord/CHANGELOG.md

+*   `index_exists?` with `:name` option does verify specified columns.
+
+    Example:
+
+        add_index :articles, :title, name: "idx_title"
+
+        # Before:
+        index_exists? :articles, :title, name: "idx_title" # => `true`
+        index_exists? :articles, :body, name: "idx_title" # => `true`
+
+        # After:
+        index_exists? :articles, :title, name: "idx_title" # => `true`
+        index_exists? :articles, :body, name: "idx_title" # => `false`
+
+    *Yves Senn*, *Matthew Draper*
+
 *   When calling `update_columns` on a record that is not persisted, the error
     message now reflects whether that object is a new record or has been
     destroyed.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -43,13 +43,14 @@ def table_exists?(table_name)
       #   index_exists?(:suppliers, :company_id, name: "idx_company_id")
       #
       def index_exists?(table_name, column_name, options = {})
-        column_names = Array(column_name)
-        index_name = options.key?(:name) ? options[:name].to_s : index_name(table_name, :column => column_names)
-        if options[:unique]
-          indexes(table_name).any?{ |i| i.unique && i.name == index_name }
-        else
-          indexes(table_name).any?{ |i| i.name == index_name }
-        end
+        column_names = Array(column_name).map(&:to_s)
+        index_name = options.key?(:name) ? options[:name].to_s : index_name(table_name, column: column_names)
+        checks = []
+        checks << lambda { |i| i.name == index_name }
+        checks << lambda { |i| i.columns == column_names }
+        checks << lambda { |i| i.unique } if options[:unique]
+
+        indexes(table_name).any? { |i| checks.all? { |check| check[i] } }
       end
 
       # Returns an array of Column objects for the table specified by +table_name+.

activerecord/test/cases/migration/index_test.rb

@@ -95,6 +95,12 @@ def test_index_exists_on_multiple_columns
         assert connection.index_exists?(:testings, [:foo, :bar])
       end
 
+      def test_index_exists_with_custom_name_checks_columns
+        connection.add_index :testings, [:foo, :bar], name: "my_index"
+        assert connection.index_exists?(:testings, [:foo, :bar], name: "my_index")
+        assert_not connection.index_exists?(:testings, [:foo], name: "my_index")
+      end
+
       def test_valid_index_options
         assert_raise ArgumentError do
           connection.add_index :testings, :foo, unqiue: true