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# Index as a Table

By default, the index page is a table with each of the models content columns and links to
show, edit and delete the object. There are many ways to customize what gets
displayed.

## Defining Columns

To display an attribute or a method on a resource, simply pass a symbol into the
column method:

```ruby
index do
  selectable_column
  column :title
end
```

For association columns we make an educated guess on what to display by
calling the following methods in the following order:

```ruby
:display_name, :full_name, :name, :username, :login, :title, :email, :to_s
```

This can be customized in `config/initializers/active_admin.rb`.

If the default title does not work for you, pass it as the first argument:

```ruby
index do
  selectable_column
  column "My Custom Title", :title
end
```

Sometimes that just isn't enough and you need to write some view-specific code.
For example, say we wanted a "Title" column that links to the posts admin screen.

`column` accepts a block that will be rendered for each of the objects in the collection.
The block is called once for each resource, which is passed as an argument to the block.

```ruby
index do
  selectable_column
  column "Title" do |post|
    link_to post.title, admin_post_path(post)
  end
end
```

To setup links to View, Edit and Delete a resource, use the `actions` method:

```ruby
index do
  selectable_column
  column :title
  actions
end
```

You can also append custom links to the default links:

```ruby
index do
  selectable_column
  column :title
  actions do |post|
    link_to "Preview", admin_preview_post_path(post), class: "member_link"
  end
end
```

Or forego the default links entirely:

```ruby
index do
  column :title
  actions defaults: false do |post|
    link_to "View", admin_post_path(post)
  end
end
```

## Sorting

When a column is generated from an Active Record attribute, the table is
sortable by default. If you are creating a custom column, you may need to give
Active Admin a hint for how to sort the table.

If a column is defined using a block, you must pass the key to turn on sorting. The key
is the attribute which gets used to sort objects using Active Record.

By default, this is the column on the resource's table that the attribute corresponds to.
Otherwise, any attribute that the resource collection responds to can be used.

```ruby
index do
  column :title, sortable: :title do |post|
    link_to post.title, admin_post_path(post)
  end
end
```

You can turn off sorting on any column by passing false:

```ruby
index do
  column :title, sortable: false
end
```

## Associated Sorting

You're normally able to sort columns alphabetically, but by default you
can't sort by associated objects. Though with a few simple changes, you can.

Assuming you're on the Books index page, and Book has_one Publisher:

```ruby
controller do
  def scoped_collection
    super.includes :publisher # prevents N+1 queries to your database
  end
end
```

Then it's simple to sort by any Publisher attribute from within the index table:

```ruby
index do
  column :publisher, sortable: 'publishers.name'
end
```

## Showing and Hiding Columns

The entire index block is rendered within the context of the view, so you can
easily do things that show or hide columns based on the current context.

For example, if you were using CanCan:

```ruby
index do
  column :title, sortable: false
  column :secret_data if can? :manage, Post
end
```