A library designed to integrate the Dapper and graphql-dotnet projects with ease-of-use in mind and performance as the primary concern.

Dapper.GraphQL combines the ideas that come out-of-the-box with graphql-dotnet, and adds the following concepts:

1. Query builders
2. Entity mapper

Query builders are used to build dynamic queries based on what the client asked for in their GraphQL query. For example, given this GraphQL query:

query {
    people {
        id
        firstName
        lastName
    }
}

A proper query builder will generate a SQL query that looks something like this:

SELECT id, firstName, lastName
FROM Person

Using the same query builder, and given the following GraphQL:

query {
    people {
        id
        firstName
        lastName
        emails {
            id
            address
        }
        phones {
            id
            number
            type
        }
    }
}

A more complex query should be generated, something like:

SELECT
  person.Id, person.firstName, person.lastName,
  email.id, email.address,
  phone.id, phone.number, phone.type  
FROM Person person 
LEFT OUTER JOIN Email email ON person.Id = email.PersonId 
LEFT OUTER JOIN Phone phone ON person.Id = phone.PersonId

Entity mappers are used to map entities to Dapper from query results. Since a single entity can be composed of multiple rows of a query result, an entity mapper is designed to quickly merge multiple rows of output SQL into a single hierarchy of entities.

See the PersonEntityMapper.cs class in the test project for an example.

Dapper.GraphQL uses Microsoft's standard DI container, IServiceCollection, to manage all of the Dapper and GraphQL interactions. If you're developing in ASP.NET Core, you can add this to the ConfigureServices() method:

serviceCollection.AddDapperGraphQL(options =>
{
    // Add GraphQL types
    options.AddType<CompanyType>();
    options.AddType<EmailType>();
    options.AddType<PersonType>();
    options.AddType<GraphQL.PhoneType>();
    options.AddType<PersonQuery>();

    // Add the GraphQL schema
    options.AddSchema<PersonSchema>();

    // Add query builders for dapper
    options.AddQueryBuilder<Company, CompanyQueryBuilder>();
    options.AddQueryBuilder<Email, EmailQueryBuilder>();
    options.AddQueryBuilder<Person, PersonQueryBuilder>();
    options.AddQueryBuilder<Phone, PhoneQueryBuilder>();
});

When creating a SQL query based on a GraphQL query, you need 2 things to build the query properly: A query builder and entity mapper.

Each entity in a system should have its own query builder, so any GraphQL queries that interact with those entities can be automatically handled, even when nested within other entities.

In the above setup, the Email query builder looks like this:

public class EmailQueryBuilder :
    IQueryBuilder<Email>
{
    public SqlQueryContext Build(SqlQueryContext query, IHaveSelectionSet context, string alias)
    {
        // Always get the ID of the email
        query.Select($"{alias}.Id");
        // Tell Dapper where the Email class begins (at the Id we just selected)
        query.SplitOn<Email>("Id");

        var fields = context.GetSelectedFields();
        if (fields.ContainsKey("address"))
        {
            query.Select($"{alias}.Address");
        }

        return query;
    }
}

• The query represents the SQL query that's been generated so far.
• The context is the GraphQL context - it contains the GraphQL query and what data has been requested.
• The alias is what SQL alias the current table has. Since entities can be used more than once (multiple entities can have an Email, for example), it's important that our aliases are unique.

Let's break down what's happening in the Build() method:

1. query.Select($"{alias}.Id"); - select the Id of the entity. This is good practice, so that even if the GraphQL query didn't include the id, we always include it.
2. query.SplitOn<Email>("Id"); - tell Dapper that the Id marks the beginning of the Email class.
3. var fields = context.GetSelectedFields(); - Gets a list of fields that have been selected from GraphQL.
4. case "address": query.Select($"{alias}.Address"); - When address is found in the GraphQL query, add the Address to the SQL SELECT clause.

Query builders are intended to chain, as our entities tend to have a hierarchical relationship. See the PersonQueryBuilder.cs file in the test project for a good example of chaining.

Here's an example of a query definition in graphql-dotnet:

Field<ListGraphType<PersonType>>(
    "people",
    description: "A list of people.",
    resolve: context =>
    {
        // Create an alias for the 'Person' table.
        var alias = "person";
        // Add the 'Person' table to the FROM clause in SQL
        var query = SqlBuilder.From($"Person {alias}");
        // Build the query, using the GraphQL query and SQL table alias.
        query = personQueryBuilder.Build(query, context.FieldAst, alias);

        // Create a mapper that understands how to map the 'Person' class.
        var personMapper = new PersonEntityMapper();

        // Open a connection to the database
        using (var connection = serviceProvider.GetRequiredService<IDbConnection>())
        {
            // Execute the query with the person mapper
            var results = query.Execute(connection, personMapper, context.FieldAst);
            
            // `results` contains a list of people.
            return results;
        }
    }
);

The test project contains an example of how to handle this scenario. See PersonEntityMapper.cs.

See the Dapper.GraphQL.Test project for a full set of examples, including how query builders and entity mappers are designed.

To run unit tests, you must have PostgreSQL running locally on your machine. The easiest way to accomplish this is to install Docker and run PostgreSQL from the official Docker container:

From a command line, run a Postgres image from docker as follows:

docker run --name dapper-graphql-test -e POSTGRES_PASSWORD=dapper-graphql -d -p 5432:5432 postgres

Then, unit tests should function as expected.

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