If I want to create a conceptual model for my new application, and during the analysis I found that some objects like
employee,department ..etc are a part of another system. and all the data related to them like employee name or number exist in that system.
My questions here:
asp.net mvc and EF6 code-first)
OK so! your application is a "shift management app"
In essence this app doesn't really care about employees and departments. It will care about when the business day starts and ends, how many people are working at the company, how long a shift is etc.
So you should exclude them from your conceptual model. It will only complicate it. Perhaps for example, sometimes a shift is not filled by an 'Employee' as defined by the other system. Perhaps its a contract worker or some such.
Create new models for the object you do need Shift, Person, BusinessDay etc and describe the system in those terms.
When you actually come to make the system use those same objects, so that it is flexible enough to work with any data, but link them up to the other systems with a separate export or mapping layer.