I am using ADO.NET to read a bunch of data from the database into in-memory objects.
This is my domain model:
// Question.cs
public class Question
{
public int ID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public IEnumerable<Tag> Tags { get; set; }
}
// Tag.cs
public class Tag
{
public int ID { get; set; }
public string Name { get; set; }
}
On retrieving the list of Questions, I would like to fetch the related tags for each question. I am able to do this as follows:
// QuestionRepository.cs
public IList<Question> FindAll()
{
var questions = new List<Question>();
using (SqlConnection conn = DB.GetSqlConnection())
{
using (SqlCommand cmd = conn.CreateCommand())
{
cmd.CommandText = "select * from questions";
SqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
Question question = new Question();
// Populate the question object using reader
question.Load(reader);
questions.Add(question);
}
reader.Close();
}
}
return questions;
}
// Question.cs
public void Load(SqlDataReader reader)
{
ID = int.Parse(reader["ID"].ToString());
Title = reader["Title"].ToString();
Description = reader["Description"].ToString();
// Use Tag Repository to find all the tags for a particular question
Tags = tagRepository.GetAllTagsForQuestionById(ID);
}
return questions;
}
// TagRepository.cs
public List<Tag> GetAllTagsForQuestionById(int id)
{
List<Tag> tags = new List<Tag> ();
// Build sql query to retrive the tags
// Build the in-memory list of tags
return tags;
}
My question is, are there any best practices/patterns for fetching related objects from the database?
Most of the SO questions I came across for loading related data provide the solution for entity framework. There is no answer for this duplicate question.
Even though my code works, I would like to know other ways to do the same. The closest explanation I came across that's targeting my particular problem was Martin Fowler's Lazy Load pattern, which I believe, will result in following implementation:
public class Question
{
private TagRepository tagRepo = new TagRepository();
private IList<Tag> tags;
public int ID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public IEnumerable<Tag> Tags {
get
{
if (tags == null)
{
tags = tagRepo.GetAllTagsForQuestionById(ID);
}
return tags;
}
}
}
Are there any other alternatives?
If you are insisting on doing this in ADO.Net, then I would suggest using a little trick with anonymous types, LINQ, and Enumerable.Range(0,0).
First, you need to create a list of an anonymous type (or just create an actual class that maps back to your SQL statement)
var data = Enumerable.Range(0, 0).Select(x => new
{
QestionId = 0,
Title = "Question.Title",
Description = "Question.Description",
TagId = 0,
Name = "Tag.Name"
}).ToList();
Next is where you do your ADO.Net stuff query the database and get the results.
The key thing here is to write a Query that returns all of the data you're looking for in one query.
using (var conn = GetConnection())
{
using (var cmd = conn.CreateCommand())
{
//Construct a valid SQL statement that joins questions to tags
cmd.CommandText = "SELECT q.*, t.* FROM questions q JOIN tags t ON 1 = 1";
using (var reader = cmd.ExecuteReader())
{
while (reader.Read())
{
data.Add(new
{
QestionId = reader.IsDBNull(0) ? 0 : int.TryParse(reader.GetValue(0).ToString(), out var qId) ? qId : 0,
Title = reader.IsDBNull(1) ? string.Empty : reader.GetValue(1).ToString(),
Description = reader.IsDBNull(2) ? string.Empty : reader.GetValue(2).ToString(),
TagId = reader.IsDBNull(3) ? 0 : int.TryParse(reader.GetValue(3).ToString(), out var tId) ? tId : 0,
Name = reader.IsDBNull(4) ? string.Empty : reader.GetValue(4).ToString()
});
}
}
}
}
Now that you have your list fully populated with all of the rows, you just need to transform them back into the object you're looking for.
var questions = data.GroupBy(x => new {x.QestionId, x.Title, x.Description}).Select(y => new Question
{
Id = y.Key.QestionId,
Title = y.Key.Title,
Description = y.Key.Description,
Tags = y.Select(z => new Tag
{
Id = z.TagId,
Name = z.Name
})
}).ToList();
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