How can I tell what is causing commerce to throw entity wrapper exception errors? I'm attempting to repair and reconfigure a site that was created by another admin. I ran across some issues with commerce coupon that I've been able to get past with help I received here first then here (referenced for insight into the current error).

When I enter edit mode for the "Commerce Coupon Review Pane" view, I get the same ajax error message as before but now with currency_code:

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /admin/structure/views/view/commerce_coupon_review_pane/preview/checkout/ajax
StatusText: Service unavailable (with message)
ResponseText: EntityMetadataWrapperException:
Unable to get the data property currency_code as the parent data structure is not set.
in EntityStructureWrapper-&amp;gt;getPropertyValue()


I'm also redirected to an error page with same exception when I click on the checkout button.

I don't think its a bug due to the way that this issue cleared up in the previously referenced issues. It seems more likely something happened to the data. It would also seem that getting help from the issue queues would be unlikely because it would be impossible for me to come up with steps to reproduce the error. Unfortunately I don't know how to attack the problem. However, I suspect that when the page tries to load, somewhere along the way, it fails to find data it expects to find. If this is too general of a problem what steps can I take to find out more precisely what is happening?

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Update:
Thank you for the responses @pcambra, @tenken and @Clive. It appears that there is a broken relationship. I am missing these settings in the Commerce Coupon Review Pane:

"FIELDS"

 (Commerce Order referencing Commerce Coupon) Commerce Order: Order ID 

"CONTEXTUAL FILTERS"

 (Commerce Order referencing Commerce Coupon) Commerce Order: Order ID

"RELATIONSHIPS"

 Entity Reference: Referencing entity


When I try to add them into the view, there is no option to select them.

Also, when I try to load /admin/commerce/orders/carts/ I get the same error but it does not specify a field.

Where is this relationship made and how do I now fix the relationship so they show up in the configuration dialog?

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Update:

Following instruction I am now getting these errors. Although this is progress, I turned up nothing useful on restoring broken handlers that was not specific to a particular module.

//when I click on add under FIELDS in the view this error is produced
Notice: Undefined index: base in get_base_tables()
(line 717 of view.inc).

//otherwise these errors are produced each time I try to enter the view
Missing handler: commerce_coupon
reverse_commerce_coupon_order_reference_commerce_order relationship

Notice: Undefined property: views_handler_relationship_broken::$alias
in get_result_entities() (line 1604 of views_plugin_query_default.inc).
sounds to me like the rules the old developer put in place are still firing for coupons and since you tried to downgrade from 2.x to 1.x they're firing and its killing your page ........ look for rules that might be lingering about??

More debugging can be found by installing devel, finding this path in the sourcecode of commerce_coupon /admin/structure/views/view/commerce_coupon_review_pane/preview/checkout/ajax and using dd() on func_get_args() throughout that pages' page callback function to debug where php is trying to get currency_code.

In general installing and hitting the page with xdebug debugger, and stepping through the code, could be very helpful too -- learn how to set some breakpoints in the debugger.