C#•2y ago

❔ Need help with a LINQ query

Hi, I could use a bit of help building a LINQ query. Assume a simple entity like this:

Event
  int EventTypeId
  int CustomerId

Event
  int EventTypeId
  int CustomerId

Then you have an IEnumerable<Event>. From this collection I want to extract all customer ids that only have eventTypeId == 1 associated with them. So if there are two events for one customerId, and those two eventTypeIds are 1 and 2 then I don't want that associated customer id. If both eventTypeIds are 1 then I want that specific customerId. Does that make sense? Currently I have this

from event in events
group event by event.CustomerId
into customerEvents
where customerEvents.All(event => event.EventTypeId != 2)
select customerEvents

from event in events
group event by event.CustomerId
into customerEvents
where customerEvents.All(event => event.EventTypeId != 2)
select customerEvents

But looking at it it doesn't really make sense. And it doesn't return what I thought it would. Any ideas?

13 Replies

Angius•2y ago

events
  .Where(e => e.EventTypeId == 1)
  .GroupBy(e => e.CustomerId)
  .Where(g => g.Count() == 1)
  .Select(g => g.Key.CustomerId);

events
  .Where(e => e.EventTypeId == 1)
  .GroupBy(e => e.CustomerId)
  .Where(g => g.Count() == 1)
  .Select(g => g.Key.CustomerId);

Something like this should work Ah, wait, you want events with typeid 2 as well

EsaOP•2y ago

Oh hang on I think I mixed up my instruction 🙂 I'll verify it, hang on a bit..

Angius•2y ago

This code only gives you ones that have EventTypeId equal to 1, and deduplicates it by customer ID

EsaOP•2y ago

events
  .Where(e => e.EventTypeId == 1)

events
  .Where(e => e.EventTypeId == 1)

Would this imply that there may exist events with EventTypeId == 2 for a given customerId, but that we just filter them out? Because if they exist in the collection, then I can't use that specific customerId

Angius•2y ago

It gets all events where EventTypeId is 1 Doesn't care about customer IDs

EsaOP•2y ago

Ok, I think we need to be more specific. A customerId may have several events associated with them. Several 1s or several 2s or a mix of both. If there is any 2 at all, then all of the 1s for that customer id are unusable.

Angius•2y ago

Ah, gotcha You'd need to group by user ID first, then

EsaOP•2y ago

Ah, okay

Angius•2y ago

Then, in each group, check if an event with type ID 1 exists And filter accordingly

EsaOP•2y ago

events
  .GroupBy(e => e.CustomerId)
  .Where(g => g.All(e => e.EventTypeId != 2))
  .Select(g => g.Key.CustomerId);

events
  .GroupBy(e => e.CustomerId)
  .Where(g => g.All(e => e.EventTypeId != 2))
  .Select(g => g.Key.CustomerId);

So something like this?

Angius•2y ago

Something like that, yeah

ero•2y ago

i wish linq had a None, but i understand why it doesn't

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Need help with a LINQ query