Add more documentation for create_join_table.

Explain that it doesn't create indices by default and
that it also has a block form.

1 files changed, 10 insertions, 0 deletions

diff --git a/guides/source/migrations.md b/guides/source/migrations.md
index c4fbae8925..d738d847e9 100644
--- a/guides/source/migrations.md
+++ b/guides/source/migrations.md
@@ -344,6 +344,16 @@ create_join_table :products, :categories, column_options: {null: true}
 will create the `product_id` and `category_id` with the `:null` option as
 `true`.
 
+`create_join_table` also accepts a block, which you can use to add indices
+(which are not created by default) or additional columns:
+
+```ruby
+create_join_table :products, :categories do |t|
+  t.index :products
+  t.index :categories
+end
+```
+
 ### Changing Tables
 
 A close cousin of `create_table` is `change_table`, used for changing existing