Unable to create a constant value - only primitive types

First way :

Remove ToList() in the first query.

//instead of retrieving mathings List, retrieve only the productIds you need (which are a List of Primitive types)
var productIdList = db.matchings
.Where(m => m.StoreId == StoreId)
.Select(x => x.ProductId)
.ToList();

var products = db.Products
.Where(p => productIdList
           .Contains(p.ProductId))
.ToList();

//other way
var produts = db.Products
             .Where(p => db.matchings
                        .Any(m => m.StoreId == StoreId && 
                             m.ProductId == p.ProductId)
                    )
             .ToList();

Because I think you're in linq2entities, and you're using a List of Matchings in a query which is not possible (the title of your topic tend to make me believe that's your problem).

This looks like a place to use join

 var query =
    from product in db.Products
    join matching in db.Matchings
    on product.ProductId equals matching.ProductId into matchGroup
    where matchGroup.Count() > 0 and matching.StoreId == StoreId
    select product;

I was facing the same issue when writing the following query using collection and EF tables:

var result = (from listItem in list
              join dbRecord in Context.MY_TABLE
                  on listItem.MyClass.ID equals dbRecord.ID
              select new { dbRecord, listItem.SomeEnum }).ToList();

I could solve it by changing the source order in in:

var result = (from dbRecord in Context.MY_TABLE
              join listItem in list
                  on dbRecord.ID equals listItem.MyClass.ID
              select new { dbRecord, listItem.SomeEnum }).ToList();

You can try the following. It has worked for me as my ProductId os of type nullable.

IQueryable<matchings> data = db.matchings.Any(u => product.ProductId.Value == u.ProductId);

or this

IQueryable<matchings> data = db.matchings.Any(u => product.ProductId.Value.Equals(u.ProductId));

This worked in my case as the target id is a nullable type as it points to a 1:0 -> * relationship.

Unable to create a constant value - only primitive types

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