At some point Apple decided to remove the option in the Messages app to choose the location to save chat history. Ever since that change, I always seem to forget where my chat history is saved. Here is where chat history is now stored:
~/Library/Containers/com.apple.iChat/Data/Library/Messages/ArchiveUpdate
The location of the Messages history has moved in Yosemite. Here is the new location:
~/Library/MessagesThat directory contains the Messages chat history, as well as all attachments.