Advanced data access with Dapper
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Session delivered at PrDC Winnipeg 2018. Advanced .NET data access techniques using the Dapper Micro ORM
![Loading Related Objects – Multiple Queries
• Get data using multiple queries and wire them up yourself
• Still executed in a single command that returns multiple results sets
using (var multi = await connection.QueryMultipleAsync(query, new{FromCode = from} ))
{
scheduledFlights = multi.Read<ScheduledFlight>();
var airports = multi.Read<Airport>().ToDictionary(a => a.Id);
foreach(var flight in scheduledFlights)
{
flight.Airport = airports[flight.AirportId];
}
}](https://image.slidesharecdn.com/advanceddataaccesswithdapper-180614131507/85/Advanced-data-access-with-Dapper-17-320.jpg)
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Advanced data access with Dapper
- 1. Advanced Data Access with Dapper @Dave_Paquette Microsoft MVP (ASP.NET/IIS) [email protected] http://www.davepaquette.com
- 2. Data Access How did we get here? Why is this so hard? public class Foo { //…some properties } SQL Server
- 3. Data Access in .NET • In the beginning there was System.Data • IDbConnection • IDbCommand • IDataReader • IDbTransaction
- 4. Sytem.Data - Reading Records var results = new List<Employee>(); using (SqlConnection connection = new SqlConnection(Settings.ConnectionString)) { SqlCommand command = new SqlCommand(“SELECT * FROM Employees”, connection); connection.Open(); IDataReader reader = command.ExecuteReader(); while (reader.Read()) { results.Add(ReadSingleRow(reader)); } reader.Close(); } return results;
- 5. The ORMs will save us • Hide details of database • Generate SQL based on object model and configuration • ChangeTracking • Complex mapping strategies • Many-to-many relationships • Inheritance
- 7. ORM Pain Points • Black box code generation –What is going on? • Performance Problems • Eager Loading vs Lazy Loading • Complex Inheritance Chains • Dealing with disconnected entities (in a web context)
- 8. The Micro-ORMs will save us! • Query --> Objects • Usually focused on speed • Light on features
- 9. Popular Micro-ORMs • Dapper • PetaPOCO • Massive • Simple.Data
- 10. Dapper • Used in production at Stack Exchange • Super fast and easy to use
- 11. Performance of SELECT mapping over 500 iterations - POCO serialization Method Duration Hand coded (using a SqlDataReader) 47ms Dapper ExecuteMapperQuery 49ms ServiceStack.OrmLite (QueryById) 50ms PetaPoco 52ms BLToolkit 80ms SubSonic CodingHorror 107ms NHibernate SQL 104ms Linq 2 SQL ExecuteQuery 181ms Entity framework ExecuteStoreQuery 631ms https://github.com/StackExchange/dapper-dot-net#performance-of-select-mapping-over-500-iterations---dynamic-serialization
- 12. How can it be that fast? • Dapper dynamically writes code for you • Emits IL code for tasks like loading a data reader into an object • https://github.com/StackExchange/Dapper/blo b/master/Dapper/SqlMapper.cs#L3078
- 13. Dapper works on Database Connections A set of extension methods on IDbConnection Aircraft aircraft; using (var connection = new SqlConnection(_connectionString)) { await connection.OpenAsync(); //Optional var query = "SELECT * FROM Aircraft WHERE Id = @Id"; Aircraft aircraft = await connection.QuerySingleAsync<Aircraft>(query, new {Id = id}); }
- 14. Querying Simple Objects Aircraft aircraft = await connection.QuerySingleAsync<Aircraft>(query, new {Id = id}); IEnumerable<Aircraft> aircraft = await connection.QueryAsync<Aircraft>(query);
- 15. Loading Related Objects – Multi-Mapping • Write a single query that returns all the data in a single row scheduledFlights = await connection.QueryAsync<ScheduledFlight, Airport, ScheduledFlight>(query, (flight, airport) => { flight.Airport = airport; return flight; }, new{FromCode = from} );
- 16. Multi-Mapping Caveats • Data duplication • Query returns a bloated data set • Multiple instances of an object that represent the same thing • This is totally fine forOne-to-One relationships • No duplication here
- 17. Loading Related Objects – Multiple Queries • Get data using multiple queries and wire them up yourself • Still executed in a single command that returns multiple results sets using (var multi = await connection.QueryMultipleAsync(query, new{FromCode = from} )) { scheduledFlights = multi.Read<ScheduledFlight>(); var airports = multi.Read<Airport>().ToDictionary(a => a.Id); foreach(var flight in scheduledFlights) { flight.Airport = airports[flight.AirportId]; } }
- 18. Multi-Mapping vs Multiple Queries 100 ScheduledFlight Records 1,000 ScheduledFlight Records Method Mean MultiMapping 926.5 us MultipleResultSets 705.9 us Method Mean MultiMapping 5.098 ms MultipleResultSets 2.809 ms
- 19. Loading More Complex Object Graphs
- 20. Loading More Complex Object Graphs
- 21. Paging through large collections • Use an OFFSET FETCH query • Include a total count in the result SELECT * FROM Flight f INNER JOIN ScheduledFlight sf ON f.ScheduledFlightId = sf.Id ORDER BY Day, FlightNumber OFFSET 0 ROWS FETCH NEXT 10 ROWS ONLY SELECT COUNT(*) FROM Flight f INNER JOIN ScheduledFlight sf ON f.ScheduledFlightId = sf.Id
- 22. Insert / Update / Delete • Use the ExecuteAsync method • Built-in support for batch inserts await connection.ExecuteAsync( @"INSERT INTO Flight(ScheduledFlightId, Day, ScheduledDeparture, ScheduledArrival) VALUES(@ScheduledFlightId, @Day, @ScheduledDeparture, @ScheduledArrival)", flights);
- 23. Buffering •Working with really large result sets? • Consider setting buffering=false for queries
- 24. Dapper Contrib • Generates SQL for simple CRUD operations
- 25. Questions? Check out my blog posts http://bit.ly/dapperseries
Editor's Notes
- #3: Data access has consumed such a huge amount of our time. - Why is that? - What is so hard about this?
- #4: These 4 interfaces were in the original .NET framework. They have been the basis for ALL database access since then. Each database provider has an implementation of each of these interfaces (eg. SqlConnection) Every ORM or Micro-ORM written in .NET is based on these 4 interfaces.
- #5: Show sample code for reading and writing data using the raw SQL
- #6: Swinging the pendulum
- #20: Demo Start with loading simply the first level (Airport + Arrival and Departure Flights) Using multi-mapping to load the ScheduledFlight info Use multi queries to also load each ScheduledFlight’s Arrival and Departure airports