A lien, filed as prescribed in this article, may be assigned by a written instrument signed and acknowledged by the lienor, at any time before the discharge thereof. Such assignment shall contain the names and places of residence of the assignor and assignee, the amount of the lien and the date of filing the notice of lien, and be filed in the office where the notice of the lien assigned is filed. The facts relating to such an assignment and the names of the assignee shall be entered by the proper officer in the book where the notice of lien is entered and opposite the entry thereof. Unless such assignment is filed, the assignee need not be made a defendant in an action to foreclose a mortgage, lien or other incumbrance. A payment made by the owner of the real property subject to the lien assigned or by his agent or contractor, or by the contractor of a public corporation, to the original lienor, on account of such lien, without notice of such assignment and before the same is filed, shall be valid and of full force and effect. Except as prescribed herein, the validity of an assignment of a lien shall not be affected by a failure to file the same.