multi-map | Map that can contains multiple values for the same key | Map library

Lets say I have a Dynamo DB table named 'student_course'. I want to store the course that each student is taking in a University. One student can take multiple courses at a time and one course can have many student at a time. So basically it is a multi-mapping.

My data access pattern has only one use case -

1. get a record of one student and one course at a time, i.e. is get data for each StudentId and CourseId combination. Its is guaranteed that for a student-id and course-id combination, there is only one record available.

To achieve this I can store the data in these 2 ways -

1. Partition-key = {student-id}, sort-key = {course-id}
2. Partition-key = "studentId:{student-id}_courseId:{course-id}", sort-key does not exist

My Question is - Which query would perform better, if there is any difference, that is? Which one should I choose over the other and why?

I have a model that has a HashSet member. Since I wasn't able to get Dapper to automatically create the set from an array, I thought I would just use a multi-mapping query and get the array as a separate column, split on it and create the set inside a lambda. Just to be clear, I want to deserialize the whole model, not just the bit for the set.

My first multi-mapping attempt also failed, so I decided to try something simpler, get an array and deserialize it into a List or int[]. And this is where I'm stuck.

I am using a PostgreSQL 12 database.

Code snippet is as follows

I am using Dapper with MySql to get a list of uTeacher with multi-mapping but I can't get it to work! I get Invalid Cast exception from Int32 to uDepartement class and other classes!!

( DataException: Error parsing column 33 (Department=1 - Int32) )

I have the following Classes:

For the code shown below, I am getting the following error message.

*** Error ***

When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id (Parameter 'splitOn')

From what i understand i am passing the splitOn parameter am i missing something obvious?

Code

I'm trying to perform a simple query and the result data is almost all null.

I have this table structure

Table Registros

I want to use the splitOn feature denoted here: https://dapper-tutorial.net/result-multi-mapping

to group every Order of the results to a integer property "EmployeeId". I Followed the advice from How to map to a Dictionary object from database results using Dapper Dot Net?

but I am getting a An item with the same key has already been added. so how can I group my orders by EmployeeId?

I cannot modify the Order class and I prefer using a dictionary over creating a class that wraps Order. However, if there is no other way I am open to the idea of wrapping Order

https://dotnetfiddle.net/hn6Sjf

I have the jvectormap of Australia. On click of a particular state in the map, it should zoom out that particular state in the same map. Is there any way to achieve this without using multi-map.

New to dapper and having a difficult time figuring out how to complete this query. I've looked at other examples on their site and SO, but I'm still having a difficult time putting things together.

I have the following stored procedure in my sql db.

New to Dapper here! Having an issue with multi-mapping. This is my query: