Unusual behavior of Shopping Cart (Drupal Commerce) I am using Drupal Commerce for a portal &amp; I am facing a issue which is re-producible on fresh instance as well.

Suppose if I have a account on a website and I am logged in. I add some items to my cart and leave. It will remain in my cart even if I login from another system.

But if I don't login from another system at start &amp; click "Add to Cart", item get added to my cart with help of session. Now if I login into system, only current items are in cart, not which I have added from another system in my account.

I mean, we loose those items which were in cart before.

This behaviour I checked on some other ecommerce website as well, but they add session cart into the account cart once you login.

What can be the solution of this ?
What you describe is the default behavior in Drupal Commerce, as it is supporting the idea of "multiple carts", so when you log in you get the items that you've got in your current cart and the other cart will be kept in the "background". 

In fact, if you end the purchase you have once logged in, you'll most probably see that the items you had in the cart in the first place are back in there.

I'm afraid this is something you can't solve without some coding or rules, the theory here is that on the user login event, you get all the past orders from that user and combine them in the current cart of the user. This is not something you normally want as the cart pulled might be old.