You need to find all orders in the processing state. You have written the code:

When you run the code, you get the following exception:

How do you resolve the exception?

Your module, MyCompany_MyModule, is using the frontName mymodule. You need to locate the class responsible for the frontend path /mymodule/custom.
What file contains the controller class for the frontend path /mymodule/custom?

In a code review of a merchant's site you have discovered that there are multiple observers for the checkout_submit_all_after event. After closer inspection, you notice one of these observers is relying on the others being executed before it is called.
What risk does this pose, and how can it be mitigated?

How do you add a foreign key to an existing table created by another module?

You are working on a new entity called vendor. You implemented the model, resource model and collection. You want to ensure that standard model events will be fired for your model, so an observer can be created for the events vendor_save_after, vendor_save_commit_after and others.
How do you do that?