A method for assembling a mixer-container intended for receiving a biopharmaceutical fluid includes providing a container with a mixing device including at least one shaft having an adjustable length, and at least one first bearing attached to a wall of the container, the shaft extending at least into the inner space from the first bearing; a rigid outer device that compresses the container; and a drive motor located outside the container, the first bearing of the container is placed so as to be spaced apart from the motor, and the length of the shaft is adjusted along the main axis by arranging the shaft opposite the motor to enable the motor to rotate the shaft.