why does the texture gets stretched on my rectangle shaped object? I have UV Unwrapped my rectangle looking object.
Even though the UV looks fine to me, the texture gets stretched.

I tried using heavier subdivide to make each face more like a square, I tried different seam patterns but I couldn't find the solution.

I noticed that when I Scale an object, the texture gets stretched, so I created a new squared and extruded it from one side to make a rectangle instead of scaling it to the side, but still same thing happened, texture got streched.

I also tried googling this question, but couldn't find a tutorial talks about unwrapping a rectangle, this might be because it's so easy so no one creates a tutorial about this.
but anyways I finally made the decision to ask myself in here.

How can I fix this?

Below you can find a screenshot of my object, seams and the UV.



Best regards, 
Amirreza
The stretching is happening because your model's polygons are rectangular, but the UVs are square. Apply Scale to the object before UV unwrapping.

To apply scale, select your object in Object mode, then go to Object > Apply > Scale (or CTRL + A > Scale)