I purchased a flight from Denver through Chicago to Venice. The portion of the flight from Chicago to Venice is on American Airlines code-share partner, Iberia Airlines.
After I paid for my flight and was locked into the reservation, I discovered that American and Iberia do not share seat assignment information, and that I could not get my seat until the date of the flight three months later at the Iberia Airlines gate. Had I known about this ahead of time I would not have chosen American. A 17-hour flight is too long to gamble on an uncomfortable seat, and I should have been able to get my seat when I purchased the flight.
I cancelled the reservation and booked with another airline.
American Airlines should either share seat arrangements with Iberia or warn prospective fliers booking a code-share flight that they will not be able to obtain a seat assignment until the last minute.