How to model relationship using event sourcing?

[Leo Zhang]

In our scenario, we have a Course entity to represent course content. For each student attending a course, there is a CourseSession entity representing the learning progress of the student in the course. So there is a one-to-many relationship between Course and CourseSession. If using relational database, there will be a course table and course_session table, in which course has a unique ID and course session is uniquely identified by (courseId + studentId). We try to model this using event sourcing, and our event table is like following

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| entity_type | entity_id | event_type | event_data |

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this is fine for storing course, there is a courseId we can use as entity_id. But for CourseSession, there isn't an intrinsic id attribute, we have to use the concatenation of (courseId + studentId) as entity_id, which is not quite natural. Is there a better way to model this kind of relationship?

[mynkow]

Actually what you have suggested is how I do it all the time and it is very natural.

Usually I use URNs for identifiers. => https://tools.ietf.org/html/rfc5141

Here is an example:

CourseId => urn:boundedcontext:course:123

CourseSessionId  => urn:boundedcontext:course:123/student:456

[Nils Kilden-Pedersen]

Seems to me that CourseSession is just another entity (AR) which needs its own identifier, same as with Course and Student.

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