given a list of ids, I can query all relevant rows by:
context.Table.Where(q => listOfIds.Contains(q.Id));
But how do you achieve the same functionality when the Table has a composite key?
This is a nasty problem for which I don't know any elegant solution.
Suppose you have these key combinations, and you only want to select the marked ones (*).
Id1 Id2
--- ---
1 2 *
1 3
1 6
2 2 *
2 3 *
... (many more)
How to do this is a way that Entity Framework is happy? Let's look at some possible solutions and see if they're any good.
Solution 1: Join (or Contains) with pairs
The best solution would be to create a list of the pairs you want, for instance Tuples, (List<Tuple<int,int>>) and join the database data with this list:
from entity in db.Table // db is a DbContext
join pair in Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity
In LINQ to objects this would be perfect, but, too bad, EF will throw an exception like
Unable to create a constant value of type 'System.Tuple`2 (...) Only primitive types or enumeration types are supported in this context.
which is a rather clumsy way to tell you that it can't translate this statement into SQL, because Tuples is not a list of primitive values (like int or string). For the same reason a similar statement using Contains (or any other LINQ statement) would fail.
Solution 2: In-memory
Of course we could turn the problem into simple LINQ to objects like so:
from entity in db.Table.AsEnumerable() // fetch db.Table into memory first
join pair Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity
Needless to say that this is not a good solution. db.Table could contain millions of records.
Solution 3: Two Contains statements (incorrect)
So let's offer EF two lists of primitive values, [1,2] for Id1 and [2,3] for Id2. We don't want to use join, so let's use Contains:
from entity in db.Table
where ids1.Contains(entity.Id1) && ids2.Contains(entity.Id2)
select entity
But now the results also contains entity {1,3}! Well, of course, this entity perfectly matches the two predicates. But let's keep in mind that we're getting closer. In stead of pulling millions of entities into memory, we now only get four of them.
Solution 4: One Contains with computed values
Solution 3 failed because the two separate Contains statements don't only filter the combinations of their values. What if we create a list of combinations first and try to match these combinations? We know from solution 1 that this list should contain primitive values. For instance:
var computed = ids1.Zip(ids2, (i1,i2) => i1 * i2); // [2,6]
and the LINQ statement:
from entity in db.Table
where computed.Contains(entity.Id1 * entity.Id2)
select entity
There are some problems with this approach. First, you'll see that this also returns entity {1,6}. The combination function (a*b) does not produce values that uniquely identify a pair in the database. Now we could create a list of strings like ["Id1=1,Id2=2","Id1=2,Id2=3]" and do
from entity in db.Table
where computed.Contains("Id1=" + entity.Id1 + "," + "Id2=" + entity.Id2)
select entity
(This would work in EF6, not in earlier versions).
This is getting pretty messy. But a more important problem is that this solution is not sargable, which means: it bypasses any database indexes on Id1 and Id2 that could have been used otherwise. This will perform very very poorly.
Solution 5: Best of 2 and 3
So the most viable solution I can think of is a combination of Contains and a join in memory: First do the contains statement as in solution 3. Remember, it got us very close to what we wanted. Then refine the query result by joining the result as an in-memory list:
var rawSelection = from entity in db.Table
where ids1.Contains(entity.Id1) && ids2.Contains(entity.Id2)
select entity;
var refined = from entity in rawSelection.AsEnumerable()
join pair in Tuples on new { entity.Id1, entity.Id2 }
equals new { Id1 = pair.Item1, Id2 = pair.Item2 }
select entity;
It's not elegant, messy all the same maybe, but so far it's the only scalable1 solution to this problem I found, and applied in my own code.
Solution 6: Build a query with OR clauses
Using a Predicate builder like Linqkit or alternatives, you can build a query that contains an OR clause for each element in the list of combinations. This could be a viable option for really short lists. With a couple of hundreds of elements, the query will start performing very poorly. So I don't consider this a good solution unless you can be 100% sure that there will always be a small number of elements. One elaboration of this option can be found here.
Solution 7: Unions
There's also a solution using UNIONs that I posted later here.
1As far as the Contains statement is scalable: Scalable Contains method for LINQ against a SQL backend
Solution for Entity Framework Core with SQL Server
🎉 NEW! QueryableValues EF6 Edition has arrived!
The following solution makes use of QueryableValues. This is a library that I wrote to primarily solve the problem of query plan cache pollution in SQL Server caused by queries that compose local values using the Contains LINQ method. It also allows you to compose values of complex types in your queries in a performant way, which will achieve what's being asked in this question.
First you will need to install and set up the library, after doing that you can use any of the following patterns that will allow you to query your entities using a composite key:
// Required to make the AsQueryableValues method available on the DbContext.
using BlazarTech.QueryableValues;
// Local data that will be used to query by the composite key
// of the fictitious OrderProduct table.
var values = new[]
{
new { OrderId = 1, ProductId = 10 },
new { OrderId = 2, ProductId = 20 },
new { OrderId = 3, ProductId = 30 }
};
// Optional helper variable (needed by the second example due to CS0854)
var queryableValues = dbContext.AsQueryableValues(values);
// Example 1 - Using a Join (preferred).
var example1Results = dbContext
.OrderProduct
.Join(
queryableValues,
e => new { e.OrderId, e.ProductId },
v => new { v.OrderId, v.ProductId },
(e, v) => e
)
.ToList();
// Example 2 - Using Any (similar behavior as Contains).
var example2Results = dbContext
.OrderProduct
.Where(e => queryableValues
.Where(v =>
v.OrderId == e.OrderId &&
v.ProductId == e.ProductId
)
.Any()
)
.ToList();
Useful Links
Nuget Package
GitHub Repository
Benchmarks
QueryableValues is distributed under the MIT license.
You can use Union for each composite primary key:
var compositeKeys = new List<CK>
{
new CK { id1 = 1, id2 = 2 },
new CK { id1 = 1, id2 = 3 },
new CK { id1 = 2, id2 = 4 }
};
IQuerable<CK> query = null;
foreach(var ck in compositeKeys)
{
var temp = context.Table.Where(x => x.id1 == ck.id1 && x.id2 == ck.id2);
query = query == null ? temp : query.Union(temp);
}
var result = query.ToList();
You can create a collection of strings with both keys like this (I am assuming that your keys are int type):
var id1id2Strings = listOfIds.Select(p => p.Id1+ "-" + p.Id2);
Then you can just use "Contains" on your db:
using (dbEntities context = new dbEntities())
{
var rec = await context.Table1.Where(entity => id1id2Strings .Contains(entity.Id1+ "-" + entity.Id2));
return rec.ToList();
}
You need a set of objects representing the keys you want to query.
class Key
{
int Id1 {get;set;}
int Id2 {get;set;}
If you have two lists and you simply check that each value appears in their respective list then you are getting the cartesian product of the lists - which is likely not what you want. Instead you need to query the specific combinations required
List<Key> keys = // get keys;
context.Table.Where(q => keys.Any(k => k.Id1 == q.Id1 && k.Id2 == q.Id2));
I'm not completely sure that this is valid use of Entity Framework; you may have issues with sending the Key type to the database. If that happens then you can be creative:
var composites = keys.Select(k => p1 * k.Id1 + p2 * k.Id2).ToList();
context.Table.Where(q => composites.Contains(p1 * q.Id1 + p2 * q.Id2));
You can create an isomorphic function (prime numbers are good for this), something like a hashcode, which you can use to compare the pair of values. As long as the multiplicative factors are co-prime this pattern will be isomorphic (one-to-one) - i.e. the result of p1*Id1 + p2*Id2 will uniquely identify the values of Id1 and Id2 as long as the prime numbers are correctly chosen.
But then you end up in a situation where you're implementing complex concepts and someone is going to have to support this. Probably better to write a stored procedure which takes the valid key objects.
Ran into this issue as well and needed a solution that both did not perform a table scan and also provided exact matches.
This can be achieved by combining Solution 3 and Solution 4 from Gert Arnold's Answer
var firstIds = results.Select(r => r.FirstId);
var secondIds = results.Select(r => r.SecondId);
var compositeIds = results.Select(r => $"{r.FirstId}:{r.SecondId}");
var query = from e in dbContext.Table
//first check the indexes to avoid a table scan
where firstIds.Contains(e.FirstId) && secondIds.Contains(e.SecondId))
//then compare the compositeId for an exact match
//ToString() must be called unless using EF Core 5+
where compositeIds.Contains(e.FirstId.ToString() + ":" + e.SecondId.ToString()))
select e;
var entities = await query.ToListAsync();
For EF Core I use a slightly modified version of the bucketized IN method by EricEJ to map composite keys as tuples. It performs pretty well for small sets of data.
Sample usage
List<(int Id, int Id2)> listOfIds = ...
context.Table.In(listOfIds, q => q.Id, q => q.Id2);
Implementation
public static IQueryable<TQuery> In<TKey1, TKey2, TQuery>(
this IQueryable<TQuery> queryable,
IEnumerable<(TKey1, TKey2)> values,
Expression<Func<TQuery, TKey1>> key1Selector,
Expression<Func<TQuery, TKey2>> key2Selector)
{
if (values is null)
{
throw new ArgumentNullException(nameof(values));
}
if (key1Selector is null)
{
throw new ArgumentNullException(nameof(key1Selector));
}
if (key2Selector is null)
{
throw new ArgumentNullException(nameof(key2Selector));
}
if (!values.Any())
{
return queryable.Take(0);
}
var distinctValues = Bucketize(values);
if (distinctValues.Length > 1024)
{
throw new ArgumentException("Too many parameters for SQL Server, reduce the number of parameters", nameof(values));
}
var predicates = distinctValues
.Select(v =>
{
// Create an expression that captures the variable so EF can turn this into a parameterized SQL query
Expression<Func<TKey1>> value1AsExpression = () => v.Item1;
Expression<Func<TKey2>> value2AsExpression = () => v.Item2;
var firstEqual = Expression.Equal(key1Selector.Body, value1AsExpression.Body);
var visitor = new ReplaceParameterVisitor(key2Selector.Parameters[0], key1Selector.Parameters[0]);
var secondEqual = Expression.Equal(visitor.Visit(key2Selector.Body), value2AsExpression.Body);
return Expression.AndAlso(firstEqual, secondEqual);
})
.ToList();
while (predicates.Count > 1)
{
predicates = PairWise(predicates).Select(p => Expression.OrElse(p.Item1, p.Item2)).ToList();
}
var body = predicates.Single();
var clause = Expression.Lambda<Func<TQuery, bool>>(body, key1Selector.Parameters[0]);
return queryable.Where(clause);
}
class ReplaceParameterVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
/// <summary>
/// Break a list of items tuples of pairs.
/// </summary>
private static IEnumerable<(T, T)> PairWise<T>(this IEnumerable<T> source)
{
var sourceEnumerator = source.GetEnumerator();
while (sourceEnumerator.MoveNext())
{
var a = sourceEnumerator.Current;
sourceEnumerator.MoveNext();
var b = sourceEnumerator.Current;
yield return (a, b);
}
}
private static TKey[] Bucketize<TKey>(IEnumerable<TKey> values)
{
var distinctValueList = values.Distinct().ToList();
// Calculate bucket size as 1,2,4,8,16,32,64,...
var bucket = 1;
while (distinctValueList.Count > bucket)
{
bucket *= 2;
}
// Fill all slots.
var lastValue = distinctValueList.Last();
for (var index = distinctValueList.Count; index < bucket; index++)
{
distinctValueList.Add(lastValue);
}
var distinctValues = distinctValueList.ToArray();
return distinctValues;
}
In the absence of a general solution, I think there are two things to consider:
Avoid multi-column primary keys (will make unit testing easier too).
But if you have to, chances are that one of them will reduce the
query result size to O(n) where n is the size of the ideal query
result. From here, its Solution 5 from Gerd Arnold above.
For example, the problem leading me to this question was querying order lines, where the key is order id + order line number + order type, and the source had the order type being implicit. That is, the order type was a constant, order ID would reduce the query set to order lines of relevant orders, and there would usually be 5 or less of these per order.
To rephrase: If you have a composite key, changes are that one of them have very few duplicates. Apply Solution 5 from above with that.
I tried this solution and it worked with me and the output query was perfect without any parameters
using LinqKit; // nuget
var customField_Ids = customFields?.Select(t => new CustomFieldKey { Id = t.Id, TicketId = t.TicketId }).ToList();
var uniqueIds1 = customField_Ids.Select(cf => cf.Id).Distinct().ToList();
var uniqueIds2 = customField_Ids.Select(cf => cf.TicketId).Distinct().ToList();
var predicate = PredicateBuilder.New<CustomFieldKey>(false); //LinqKit
var lambdas = new List<Expression<Func<CustomFieldKey, bool>>>();
foreach (var cfKey in customField_Ids)
{
var id = uniqueIds1.Where(uid => uid == cfKey.Id).Take(1).ToList();
var ticketId = uniqueIds2.Where(uid => uid == cfKey.TicketId).Take(1).ToList();
lambdas.Add(t => id.Contains(t.Id) && ticketId.Contains(t.TicketId));
}
predicate = AggregateExtensions.AggregateBalanced(lambdas.ToArray(), (expr1, expr2) =>
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<CustomFieldKey, bool>>
(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
});
var modifiedCustomField_Ids = repository.GetTable<CustomFieldLocal>()
.Select(cf => new CustomFieldKey() { Id = cf.Id, TicketId = cf.TicketId }).Where(predicate).ToArray();
I ended up writing a helper for this problem that relies on System.Linq.Dynamic.Core;
Its a lot of code and don't have time to refactor at the moment but input / suggestions appreciated.
public static IQueryable<TEntity> WhereIsOneOf<TEntity, TSource>(this IQueryable<TEntity> dbSet,
IEnumerable<TSource> source,
Expression<Func<TEntity, TSource,bool>> predicate) where TEntity : class
{
var (where, pDict) = GetEntityPredicate(predicate, source);
return dbSet.Where(where, pDict);
(string WhereStr, IDictionary<string, object> paramDict) GetEntityPredicate(Expression<Func<TEntity, TSource, bool>> func, IEnumerable<TSource> source)
{
var firstP = func.Parameters[0];
var binaryExpressions = RecurseBinaryExpressions((BinaryExpression)func.Body);
var i = 0;
var paramDict = new Dictionary<string, object>();
var res = new List<string>();
foreach (var sourceItem in source)
{
var innerRes = new List<string>();
foreach (var bExp in binaryExpressions)
{
var emp = ToEMemberPredicate(firstP, bExp);
var val = emp.GetKeyValue(sourceItem);
var pName = $"#{i++}";
paramDict.Add(pName, val);
var str = $"{emp.EntityMemberName} {emp.SQLOperator} {pName}";
innerRes.Add(str);
}
res.Add( "(" + string.Join(" and ", innerRes) + ")");
}
var sRes = string.Join(" || ", res);
return (sRes, paramDict);
}
EMemberPredicate ToEMemberPredicate(ParameterExpression firstP, BinaryExpression bExp)
{
var lMember = (MemberExpression)bExp.Left;
var rMember = (MemberExpression)bExp.Right;
var entityMember = lMember.Expression == firstP ? lMember : rMember;
var keyMember = entityMember == lMember ? rMember : lMember;
return new EMemberPredicate(entityMember, keyMember, bExp.NodeType);
}
List<BinaryExpression> RecurseBinaryExpressions(BinaryExpression e, List<BinaryExpression> runningList = null)
{
if (runningList == null) runningList = new List<BinaryExpression>();
if (e.Left is BinaryExpression lbe)
{
var additions = RecurseBinaryExpressions(lbe);
runningList.AddRange(additions);
}
if (e.Right is BinaryExpression rbe)
{
var additions = RecurseBinaryExpressions(rbe);
runningList.AddRange(additions);
}
if (e.Left is MemberExpression && e.Right is MemberExpression)
{
runningList.Add(e);
}
return runningList;
}
}
Helper class:
public class EMemberPredicate
{
public readonly MemberExpression EntityMember;
public readonly MemberExpression KeyMember;
public readonly PropertyInfo KeyMemberPropInfo;
public readonly string EntityMemberName;
public readonly string SQLOperator;
public EMemberPredicate(MemberExpression entityMember, MemberExpression keyMember, ExpressionType eType)
{
EntityMember = entityMember;
KeyMember = keyMember;
KeyMemberPropInfo = (PropertyInfo)keyMember.Member;
EntityMemberName = entityMember.Member.Name;
SQLOperator = BinaryExpressionToMSSQLOperator(eType);
}
public object GetKeyValue(object o)
{
return KeyMemberPropInfo.GetValue(o, null);
}
private string BinaryExpressionToMSSQLOperator(ExpressionType eType)
{
switch (eType)
{
case ExpressionType.Equal:
return "==";
case ExpressionType.GreaterThan:
return ">";
case ExpressionType.GreaterThanOrEqual:
return ">=";
case ExpressionType.LessThan:
return "<";
case ExpressionType.LessThanOrEqual:
return "<=";
case ExpressionType.NotEqual:
return "<>";
default:
throw new ArgumentException($"{eType} is not a handled Expression Type.");
}
}
}
Use Like so:
// This can be a Tuple or whatever.. If Tuple, then y below would be .Item1, etc.
// This data structure is up to you but is what I use.
[FromBody] List<CustomerAddressPk> cKeys
var res = await dbCtx.CustomerAddress
.WhereIsOneOf(cKeys, (x, y) => y.CustomerId == x.CustomerId
&& x.AddressId == y.AddressId)
.ToListAsync();
Hope this helps others.
in Case of composite key you can use another idlist and add a condition for that in your code
context.Table.Where(q => listOfIds.Contains(q.Id) && listOfIds2.Contains(q.Id2));
or you can use one another trick create a list of your keys by adding them
listofid.add(id+id1+......)
context.Table.Where(q => listOfIds.Contains(q.Id+q.id1+.......));
I tried this on EF Core 5.0.3 with the Postgres provider.
context.Table
.Select(entity => new
{
Entity = entity,
CompositeKey = entity.Id1 + entity.Id2,
})
.Where(x => compositeKeys.Contains(x.CompositeKey))
.Select(x => x.Entity);
This produced SQL like:
SELECT *
FROM table AS t
WHERE t.Id1 + t.Id2 IN (#__compositeKeys_0)),
Caveats
this should only be used where the combination of Id1 and Id2 will always produce a unique result (e.g., they're both UUIDs)
this cannot use indexes, though you could save the composite key to the db with an index
I need to make a query that return all items with the current price and the current reduction if any.
I tried a few solutions but none seem to work or respect the patterns as i understand them.
The dynamic solution:
I tried to return the data as a dynamic that would be an IQueryable where T would be (Item, CurrentItemPrice, CurrentItemStateIfAny)
public ItemRepository(CoconutEntities context) : base(context){}
public dynamic GetAllCurrentItems(){
var items = (from item in context.Items
select new {
Item = item,
CurrentItemPrice = item.ItemPrices.Where(x => item.ItemPrices.Max(y => y.EffectiveDate) == x.EffectiveDate),
CurrentItemState = item.ItemReductions.Where(x => x.StartDate <= DateTime.Now && DateTime.Now <= x.EndDate)});
return items;
}
But when i try this and i need to add filter, i can't add them the way i was expecting.
public dynamic GetCurrentItems(string filter = "", int categoryId = 1) {
dynamic result;
var categoryServices = new CategoryServices();
IEnumerable<int> categoryIdAndChildCategoriesId = categoryServices.GetCategoryIdAndChildsId(categoryId);
if (!string.IsNullOrWhiteSpace(filter))
{
result = this.GetAllCurrentItems().Where(x => ((string)(x.Item.Name)) == filter);
}
else if(categoryId != 1)
{
result = this.GetAllCurrentItems().Where(x => x.Item.ItemCategories.Any(x => categoryIdAndChildCategoriesId.Contains(x.CategoryId)));
}
return result;
}
Solution 2 : I also tried with Tuple where i should have been able to do somthing like this but i can't create Tuples from Linq to Entities if i understood in an other post. I would need to query all the item first, then use linq to object to create my tuples.
Solution 3 : I can create a viewmodel or a new model that would represent the data i need. I know this would work but i don't understand where it would stand between the two. If it is not a view model, this information won't go to the view it an other way to see an item with only the current information.
In short, there are probably many solutions to this problem, but i need help to understand which solution would be the best and why.
As I understood you you want to do as much as possible on the database - that is good. You might achieve that with tuples like that:
public IEnumerable<Tuple<Item,decimal, decimal>> GetAllCurrentItems(Expression<Func<Item, bool>> filterExpression){
using(MyContext context = new MyContext())
{
var items = context.Items
.Where(filterExpression)
.Select(item => new Tuple<Item,decimal, decimal> (
item,
item.ItemPrices.Where(x => item.ItemPrices.Max(y => y.EffectiveDate) == x.EffectiveDate),
item.ItemReductions.Where(x => x.StartDate <= DateTime.Now && DateTime.Now <= x.EndDate)});
return items;
}
}
And calling it like that:
public IEnumerable<Tuple<Item,decimal, decimal>> GetCurrentItems(string filter = "", int categoryId = 1) {
dynamic result;
var categoryServices = new CategoryServices();
IEnumerable<int> categoryIdAndChildCategoriesId = categoryServices.GetCategoryIdAndChildsId(categoryId);
if (!string.IsNullOrWhiteSpace(filter))
{
result = this.GetAllCurrentItems(x => ((string)(x.Item.Name)) == filter);
}
else if(categoryId != 1)
{
result = this.GetAllCurrentItems(x => x.Item.ItemCategories.Any(x => categoryIdAndChildCategoriesId.Contains(x.CategoryId)));
}
return result;
}
Let's say I have this method to seach my DB for products that fit a certain keyword:
public List<Product> GetByKeyword(string keyword)
{
using(var db = new DataEntities())
{
var query = db.Products.Where(x => x.Description.Contains(keyword);
return query.ToList();
}
}
This works fine, but somewhere else in my project, I want to get the active products only, still by keyword. I would like to do something like :
...
var result = ProductStore.GetByKeyword("Apple", x => x.isActive == 1);
Therefore, I created this method:
public List<Product> GetByKeyword(string keyword, Func<Product, bool> predicate = null)
{
using(var db = new DataEntities())
{
var query = db.Products.Where(x => x.Description.Contains(keyword);
if(predicate != null)
query = query.Where(x => predicate(x));
return query.ToList();
}
}
While this compiles well, the ToList() call generates a NotSupportedException because LINQ does not support the Invoke method.
Of course, I could to it with another method
i.e. GetActiveByKeyword(string keyword) but then I would have to do one for every possible variation, including the ones I didn't think of...
How do I get this to work? Thanks!
Isn't it just this:
if(predicate != null)
query = query.Where(predicate);
it's just as AD.Net says the reason why it works with Expression before is because if you say that the compiler knows it would be a lambda expression
I have 2 tables: POHeader and PODetail. I want to return all POHeaders that have an associated PODetail.ItemId = intItemId. How can I do this in LINQ?
This is what I've tried.
First I have a method in my Repository that uses the Include parameter to include the PODetails:
public IQueryable<POHeader> SearchForWithDetails(int intFacilityId)
{
return DbSet.Include("PODetails").Where(x => x.FacilityId == intFacilityId);
}
Then the result of that gets passed to:
public IQueryable<POHeader> SearchForPODetailsByItemId(IQueryable<POHeader> poHeaders, int intItemId)
{
//This returns a type of PODetail not POHeader
var q = poHeaders.SelectMany(c => c.PODetails).Where(c => c.ItemId == intItemId);
//In this case, I can't figure out the syntax :(
var p = from poHeader in poHeaders
let filteredPOs = from poDetail in poHeader.PODetails
where poDetail.ItemId == intItemId
select ????
return p;
}
What is the correct way to do this?
Also, I can foresee needing 2 results of this:
just return a IQueryable
return a joined table result.
Try this;
var result = poHeaders.Where(e => e.PODetails.Any(a => a.ItemId == intItemId));
Assuming your a Header->Detail is a 1-to-many relationship, and Detail has a navigation back to it's header called .Header:
public IQueryable<POHeader> SearchForPODetailsByItemId(IQueryable<POHeader> poHeaders, int intItemId)
{
var headersForThisItem = poHeaders.SelectMany(pod => pod.PODetails).Where(pod => pod.ItemId == intItemId)
.Select(pod=> pod.Header).Distinct();//.Distinct to eliminate duplicates when 2 Details have the same header. Not necessary if ItemId filter naturally provides distinct results.
return headersForThisItem ;
}
Untested, but I think that will give you what you want.
I was working this weekend on parallelizing a section of code using Tasks to run all the queries I needed for a dashboard page.
What I have now is many copy/paste methods with almost exactly the same query and a different line at the very end of the method.
Is there a way to write a query against one object context then detach it and pass to a method?
I want to do something like this:
using(DbContext db = new DbContext)
{
var query = db.cars.where(x => x.make == "Ford");
int handleCounts = getDoorHandleCounts(query);
}
public int getDoorHandleCounts(type? query)
{
using(DbContext db = new DbContext())
{
return query.where(x => x.partType == "DoorHandle").Count();
}
}
Any ideas?
Keep in mind all my count() methods are launched from a Task array so they'll be running in parallel. I need a new object context to run each count query on.
I've done some googling and thought about trying to use a pre-compiled query and call it from different object context's, but my real query is kind of complex with allot of if blocks to determine the where condition. Can you compile a query that isn't really simple?
You can't detach and attach a query from/to a context. However, you could reuse the first expression:
Expression<Func<Car,bool>> InitialSelection()
{
return x => x.make == "Ford";
}
public int GetDoorHandleCounts()
{
using(DbContext db = new DbContext())
{
return db.Cars()
.Where(InitialSelection())
.Where(x => x.partType == "DoorHandle").Count();
}
}
And in your task:
int handleCounts = getDoorHandleCounts();
This only works if the initial query is "simple", i.e. does not contain joins and predicates on joined sets that you should repeat over and over in each getX method.
As an alternative, you could initialize a context and feed it to a method that returns a query:
public IQueryable<Car> GetInitialQuery(DbContext db)
{
return db.Cars().Join(....)
.Where(x => x.make == "Ford")
.Where(....);
}
public int GetDoorHandleCounts()
{
using(DbContext db = new DbContext())
{
return GetInitialQuery(db)
.Where(x => x.partType == "DoorHandle").Count();
}
}
Maybe I'm misunderstanding the question, but wouldn't this do what you're looking for?
using(DbContext db = new DbContext)
{
var carsResult = db.cars.where(x => x.make == "Ford");
int handleCounts = getDoorHandleCounts(carsResult);
}
public int getDoorHandleCounts(IEnumerable<Car> result)
{
return result.where(x => x.partType == "DoorHandle").Count();
}
Edit: never mind, I only now saw your mention of the Task array.
Change
public int getDoorHandleCounts(type? query)
to
public int getDoorHandleCounts(IQueryable<cars> query)
And replace cars with the of objects the query will return.
Edit
I would just recommend passing value you're looking to filter on, like this:
{
...
int handleCounts = getDoorHandleCounts("Ford");
}
public int getDoorHandleCounts(string Make)
{
using(DbContext db = new DbContext())
{
return db.cars.where(x => x.make == Make && x.partType == "DoorHandle").Count();
}
}