I has a table Reading(ID:string(8),title:string(100),contents:text)
And in Reading models:

```
class Reading < ActiveRecord::Base
  self.table_name = "reading"
  self.primary_key = :ID
end

```

In reading controller

```
private
    # Use callbacks to share common setup or constraints between actions.
    def set_reading
      @reading = Reading.find(params[:ID])
    end

```

I have a record of reading table with ID is "20150910".

But `Reading.find("20150910")` not work.