to supports anything that responds to call (#72)

README.md

@@ -187,6 +187,27 @@ Like very advanced stuff? We grant you access to the [`column`](https://github.c
   end
 ```
 
+Note that `to:` accepts anything that responds to call and take 1, 2 or
+3 arguments.
+
+```ruby
+class ImportUserCSV
+  include CSVImporter
+
+  model User
+
+  column :birth_date, to: DateTransformer
+  column :renewal_date, to: DateTransformer
+  column :next_renewal_at, to: ->(value) { Time.at(value.to_i) }
+end
+
+class DateTransformer
+  def self.call(date)
+    Date.strptime(date, '%m/%d/%y')
+  end
+end
+```
+
 Now, what if the user does not provide the email column? It's not worth
 running the import, we should just reject the CSV file right away.
 That's easy:
@@ -206,7 +227,6 @@ import.report.status # => :invalid_header
 import.report.message # => "The following columns are required: 'email'"
 ```
 
-
 ### Update or Create
 
 You often want to find-and-update-or-create when importing a CSV file.

lib/csv_importer/row.rb

@@ -52,18 +52,19 @@ def set_attributes(model)
     # Set the attribute using the column_definition and the csv_value
     def set_attribute(model, column, csv_value)
       column_definition = column.definition
-      if column_definition.to && column_definition.to.is_a?(Proc)
-        to_proc = column_definition.to
+      transformer = column_definition.to
+      if transformer.respond_to?(:call)
+        arity = transformer.is_a?(Proc) ? transformer.arity : transformer.method(:call).arity
 
-        case to_proc.arity
+        case arity
         when 1 # to: ->(email) { email.downcase }
-          model.public_send("#{column_definition.name}=", to_proc.call(csv_value))
+          model.public_send("#{column_definition.name}=", transformer.call(csv_value))
         when 2 # to: ->(published, post) { post.published_at = Time.now if published == "true" }
-          to_proc.call(csv_value, model)
+          transformer.call(csv_value, model)
         when 3 # to: ->(field_value, post, column) { post.hash_field[column.name] = field_value }
-          to_proc.call(csv_value, model, column)
+          transformer.call(csv_value, model, column)
         else
-          raise ArgumentError, "`to` proc can only have 1, 2 or 3 arguments"
+          raise ArgumentError, "arity: #{transformer.arity.inspect} - `to` can only have 1, 2 or 3 arguments"
         end
       else
         attribute = column_definition.attribute

spec/csv_importer_spec.rb

@@ -61,14 +61,18 @@ def self.store
   class ImportUserCSV
     include CSVImporter
 
+    class ConfirmedProcessor
+      def self.call(confirmed, model)
+        model.confirmed_at = confirmed == "true" ? Time.new(2012) : nil
+      end
+    end
+
     model User
 
     column :email, required: true, as: /email/i, to: ->(email) { email.downcase }
     column :f_name, as: :first_name, required: true
     column :last_name,  to: :l_name
-    column :confirmed,  to: ->(confirmed, model) do
-      model.confirmed_at = confirmed == "true" ? Time.new(2012) : nil
-    end
+    column :confirmed,  to: ConfirmedProcessor
     column :extra, as: /extra/i, to: ->(value, model, column) do
       model.custom_fields[column.name] = value
     end
@@ -495,7 +499,7 @@ class ImportUserCSVByFirstName
     import = ImportUserCSV.new(content: csv_content).run!
 
     expect(import).to_not be_success
-    expect(import.message).to eq "Unclosed quoted field on line 3."
+    expect(import.message).to include "Unclosed quoted field"
   end
 
   it "matches columns via regexp" do