Dapper Query One To Many Relation: A Deep Dive into Mapping and Deserialization

Introduction

Dapper is a popular ORM (Object-Relational Mapping) tool for .NET developers. It provides a simple, efficient, and easy-to-use interface for interacting with databases. In this article, we will explore one of the most common challenges in Dapper: mapping queries to models with one-to-many relations.

The problem arises when we try to map a query that joins multiple tables into a single model. Specifically, let’s consider an example where each TaxId has many Merchants, and we want to map this relation using Dapper.

The Problem

Suppose we have the following database schema:

And the following data:

We want to map this data into the following model:

public class LeadModel
{
    public string TaxId { get; set; }
    public IEnumerable<string> Merchants { get; set; }
    public string CardBrand { get; set; }
}

However, when we run the query using Dapper, the Merchants property is empty. This is because Dapper is unable to map the join between the TaxId column and the MerchantId column into a single model.

The Solution

To solve this problem, we need to understand how Daper handles one-to-many relations. According to the Dapper documentation, when using the Query method with a one-to-many relation, we need to specify the join columns and use the splitOn parameter.

The corrected query would be:

var leadModels = _dbConnection.Query<string, string, string, LeadModel>(
    sql,
    (taxId, merchant, cardBrand) =>
    {
        LeadModel leadModel = new LeadModel();

        leadModel.TaxId = taxId;
        leadModel.Merchants = JsonConvert.DeserializeObject<List<string>>(merchant);
        leadModel.CardBrand = cardBrand;

        return leadModel;
    },
    dynamicParameters,
    splitOn: "TaxId,Merchant,CardBrand")
    .ToList();

In this corrected query:

• We specify the join columns TaxId, MerchantId, and CardBrand using the splitOn parameter.
• We use a lambda expression to create a new instance of the LeadModel class for each row returned by the query.
• Inside the lambda expression, we deserialize the merchant column into a list of strings using JsonConvert.DeserializeObject.

Conclusion

In conclusion, mapping queries to models with one-to-many relations in Dapper requires careful consideration of the join columns and the use of the splitOn parameter. By understanding how Dapper handles these types of relationships, we can write efficient and effective queries that return the desired data.

Note: The JsonConvert.DeserializeObject method is used to deserialize the merchant column into a list of strings. This assumes that the data in the MerchantId column is represented as a string in the database. If this is not the case, you may need to use a different deserialization approach.

Additional Considerations

When working with one-to-many relations in Dapper, there are several additional considerations to keep in mind:

• Make sure that the join columns are correctly specified to ensure that the data is mapped correctly.
• Use the splitOn parameter to specify the join columns and avoid ambiguous joins.
• When deserializing the merchant column, make sure to use a method that matches the expected data type. In this example, we used JsonConvert.DeserializeObject to deserialize the string column into a list of strings.

By following these guidelines and best practices, you can write efficient and effective queries that return the desired data when working with one-to-many relations in Dapper.

Last modified on 2024-03-16