I want to build a report which shows who is current assignee of the task and who was previous owner of the task.

Our Requirement was to re-assign task to original task assignee group. And below is the process how I implemented it.
So I created a paging grid and this grid uses ProcessAnalytics function to returns list of tasks using Process as context. On Paging grid user can select tasks they want to re-assign. Once we get list of all such tasks including other attributes we call Task Smart Service plugin(Re-assign Task Smart Service) as MNI node by passing task Id as input and original Assignee as second input and this service does the job of re-assignment. After this we store all the data like task name, task id, new assignee, old owner in database and display audit history in a grid.

Our Requirement was to re-assign task to original task assignee group. And below is the process how I implemented it.
So I created a paging grid and this grid uses ProcessAnalytics function to returns list of tasks using Process as context. On Paging grid user can select tasks they want to re-assign. Once we get list of all such tasks including other attributes we call Task Smart Service plugin(Re-assign Task Smart Service) as MNI node by passing task Id as input and original Assignee as second input and this service does the job of re-assignment. After this we store all the data like task name, task id, new assignee, old owner in database and display audit history in a grid.