Linq2db: effective way to query hierarchy of objects

问题

I'm using a c# and linq2db and have the following class/tables hierarchy:

public class YearlyTemplate
{
    [Column]
    public int Id { get; set; }
    public List<MonthlyTemplate> MonthlyTemplates { get; set;}
}

public class MonthlyTemplate
{
    [Column]
    public int Id { get; set; }

    [Column]
    public int YearlyTemplateId { get; set; }

    public YearlyTemplate YearlyTemplate{ get; set; }

    public List<DailyTemplate> DailyTemplates { get; set;}
}

public class DailyTemplate
{
    [Column]
    public int Id { get; set; }

    [Column]
    public int MonthlyTemplateId { get; set; }
    
    public MonthlyTemplate MonthlyTemplate { get; set; }
}

public class AppDataConnect : DataConnection
{
    public ITable<YearlyTemplate> YearlyTemplates => GetTable<YearlyTemplate>();
    public ITable<WeeklyTemplate> WeeklyTemplates => GetTable<WeeklyTemplate>();
    public ITable<DailyTemplate>  DailyTemplates => GetTable<DailyTemplate>();
}

I want to get a specific year from the database using where statement, but I want to get all nested MonthlyTemplates for it, and all DailyTemplates for each Monthlytemplate. How can I do it using linq2db effectively? I suppose I should use group by, but it works only on one level depth.

回答1:

Nothing special here. Just like in EF Core, linq2db contains methods for Eager Loading. At first you have to define Associations

public class YearlyTemplate
{
    [Column]
    public int Id { get; set; }

    [Association(ThisKey = nameof(YearlyTemplate.Id), OtherKey = nameof(MonthlyTemplate.YearlyTemplateId))]
    public List<MonthlyTemplate> MonthlyTemplates { get; set;}
}

public class MonthlyTemplate
{
    [Column]
    public int Id { get; set; }

    [Column]
    public int YearlyTemplateId { get; set; }

    public YearlyTemplate YearlyTemplate{ get; set; }

    [Association(ThisKey = nameof(MonthlyTemplate.Id), OtherKey = nameof(DailyTemplate.MonthlyTemplateId))]
    public List<DailyTemplate> DailyTemplates { get; set;}
}

And query

var query = 
  from y in db.YearlyTemplates
           .LoadWith(yt => yt.MonthlyTemplates)
              .ThenLoad(mt => mt.DailyTemplates)
  where y.Id == 1
  select y;

var result = query.ToArray();

Or with filters (two ways how to customize LoadWith/ThenLoad)

var query = 
  from y in db.YearlyTemplates
           .LoadWith(yt => yt.MonthlyTemplates.Where(mt => !mt.IsDeleted))
              .ThenLoad(mt => mt.DailyTemplates, q => q.Where(ti => !dt.IsDeleted))
  where y.Id == 1
  select y;

var result = query.ToArray();

Or you can use custom projection, which can be more performant because you can choose only needed fields:

var query = 
  from y in db.YearlyTemplates
  where y.Id == 1
  select new 
  {
     Id = y.Id,
     MonthlyTemplates = y.MonthlyTemplates.Select(mt => new {
        mt.Id,
        DailyTemplates = mt.DailyTemplates.ToArray()
     }).ToArray()
  };

var result = query.ToArray();

来源：https://stackoverflow.com/questions/63150864/linq2db-effective-way-to-query-hierarchy-of-objects

标签

postgresql

linq2db