Add Column#value_for to extract values

This helps us customize how to get different values for each column in a
table without having to define special renderers.

README.md

@@ -254,7 +254,7 @@ And then, on `app/views/products/_product.html.erb`:
 ``` erb
 <tr>
   <% columns.each do |column| %>
-    <td><%= product.public_send(column.name) %></td>
+    <td><%= column.value_for(product) %></td>
   <% end %>
 </tr>
 ```
@@ -309,6 +309,36 @@ separately:
 </tr>
 ```
 
+#### Extracting values for a column/row pair
+
+You can use the `Column#value_for(row)` method to extract the value for a column
+given a row. For example, in your column renderer, you might do
+
+``` erb
+<td>
+  <%= column.value_for(row) %>
+</td>
+```
+
+The behavior of `#value_for` is controlled by the Column's `accessor` option. By
+default, it will try to `public_send` the column's `name` to the row object.
+
+``` ruby
+class ProductsQuery < ApplicationQuery
+  # In this case, `column.value_for(row)` is equivalent to `row.name`
+  column :name
+
+  # Here, we're assuming the model defines a `#price` method (e.g. via
+  # `composed_of` that returns something that's better to display than
+  # the number of cents)
+  column :price_in_cents, accessor: :price
+
+  # By using a Proc as the accessor, we can do more advanced formatting of the
+  # value, without having to do anything special when rendering the cell.
+  column :purchases_count, accessor: ->(row) { "purchase".pluralize(row.purchases_count) }
+end
+```
+
 ### Internationalization
 [i18n]: #internationalization
 

lib/dtb/column.rb

@@ -51,6 +51,13 @@ class Column < QueryBuilder
     #   at the end of each row, or a checkbox at the start.
     option :database, default: true, required: true
 
+    # @!attribute [rw] accessor
+    #   The accessor to use to extract the value for this column for a given
+    #   row. Accepts a Symbol, String, or Proc. If given a Symbol or String, it
+    #   will attempt to call that method on the row object. If given a Proc, it
+    #   will call the Proc with the row object as its argument.
+    option :accessor, default: ->(row) { row.public_send(name) if database? }
+
     # @!endgroup
 
     # Looks up the column's header in the i18n sources. If the column is
@@ -73,6 +80,16 @@ def header
       i18n_lookup(:columns, default: "")
     end
 
+    # Extracts the value from the row using what you've configured as an
+    # {#accessor}.
+    def value_for(row)
+      if options[:accessor].respond_to?(:to_proc)
+        instance_exec(row, &options[:accessor].to_proc)
+      elsif options[:accessor].respond_to?(:to_sym) && row.respond_to?(options[:accessor].to_sym)
+        row.public_send(options[:accessor].to_sym)
+      end
+    end
+
     # @return [Boolean] whether this column actually affects the query or not.
     # @see #database
     def database?

test/column_test.rb

@@ -78,6 +78,25 @@ def test_can_resolve_headers_hierarchically
     assert_equal "Base Bar", bar.header
   end
 
+  def test_accesor
+    row = Struct.new(:value).new(42)
+
+    as_proc = DTB::Column.new(:value, accessor: ->(row) { row.value })
+    assert_equal 42, as_proc.value_for(row)
+
+    as_symbol = DTB::Column.new(:value, accessor: :value)
+    assert_equal 42, as_symbol.value_for(row)
+
+    as_string = DTB::Column.new(:value, accessor: "value")
+    assert_equal 42, as_string.value_for(row)
+
+    default = DTB::Column.new(:value)
+    assert_equal 42, default.value_for(row)
+
+    default_when_not_database = DTB::Column.new(:value, database: false)
+    assert_nil default_when_not_database.value_for(row)
+  end
+
   def test_rendering
     column = DTB::Column.new(:value) { |scope| scope }
     assert_nil column.renderer