How Does Dapper's Multimapping Handle Splitting Database Query Results into Multiple Objects?

Multimapping in Dapper

When using multimapping with Dapper, you aim to map a database query result to multiple objects or object types.

Correct Usage of Multimapping

To properly utilize multimapping in Dapper:

• Define your object classes with properties that match the table columns.


• Specify the "Table" attribute on the classes representing database tables. (Example: [Table("Product")])


• Use the Query method with the appropriate generic type arguments to map the results to your object classes. (Example: Query())

In your code, you had to specify the complete column list for the "splitOn" parameter because you were trying to map a single object to multiple objects. Multimapping assumes that each distinct object type starts at a different column in the result set.

Split Point

The "splitOn" parameter defines the split point where the mapping of one object ends, and the mapping of the next object begins. By default, this is set to the "Id" property of the object.

For example, in the following table:

ProductID | ProductName | AccountOpened | CustomerId | CustomerName
---------------------------------------   -------------------------

The default "splitOn" value of "CustomerId" would indicate that the "ProductItem" object mapping ends at "CustomerId," and the "Customer" object mapping begins at "CustomerId."

Corollary

It's important to note that the ordering of columns in the result set must be consistent with the expected mapping. If the ordering of "CustomerId" and "CustomerName" is reversed in the table, you will need to adjust the "splitOn" parameter to compensate, or it will result in null values.

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