LINQ to Entities 查询多对多表

Question

I am trying to implement some filtering functionality from a many to many table where a user filters on one or more tags. I currently have the following query (simplified from the version I'm working with):

var query = from adm in admissionContext.SampleTable
        from tag in admissionContext.SampleTags.Where(t => t.SampleID == adm.ID).DefaultIfEmpty()
        from tagItem in admissionContext.SampleTagElements.Where(i => i.ID == tag.ElementID).DefaultIfEmpty()
        where searchFilter.TagIDs.Count == 0 || searchFilter.TagIDs.Contains(tag.ElementID)
        select new SampleInfo
        {
            ID = adm.ID,
            ReferralTags = tagItem != null ? tagItem.TagText : string.Empty,
        };

The filtering works fine, and there is additional logic that groups the result by their SampleTableID to merge the results together, but if there are multiple tags against a SampleTable record, only one (the result from the Contains check) is retrieved and displayed in the UI, but I want it to return all related SampleTags under the same SampleID as well. but I'm not sure what the LINQ to entities syntax is to go about doing this.

This is part of a larger query so ideally I need to stick with LINQ to entities.

Answer 1

Seems like you are joining 3 tables, here is 2 approaches -

1. You can join your tables by join keyword, then use GroupBy() on that.
2. You can do by passing column values as primary key into other tables foreign key in dictionary form and then apply join.

Hope that works fine to you.

**Suggestion - Please try to avoid complex query linq is slow than lambda expressions.