Raised when a primary key is needed, but not specified in the schema or model.

Methods
N
Attributes
[R] model
Class Public methods
new(model)

Source: show | on GitHub 

# File activerecord/lib/active_record/errors.rb, line 222
def initialize(model)
  super("Unknown primary key for table #{model.table_name} in model #{model}.")
  @model = model
end