Dynamically change LINQ to Entity query
int year = 2009; // get summ of TONS2009 column
var query = from ODInfo in DataContext.CIMS_TRUCKS
where pLocationIDs.Contains(ODInfo.OID)
group ODInfo by ODInfo.OID into g
select new
{
OID = g.Key,
TotalTons = g.Sum( ODInfo => ODInfo.TONS2009)
};
In the expression "ODInfo => ODInfo.TONS2009", how do I change TONS2009 to TONS2010 or TONS2011 based on the method parameter "int year"?
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5 answers
K06a's answer is close, but won't work server side. Try the following:
IEnumerable<OutputType> myQuery(IEnumerable<InputType> data, Expression<Func<InputType,decimal>> expr)
{
return from ODInfo in DataContext.CIMS_TRUCKS
where pLocationIDs.Contains(ODInfo.OID)
group ODInfo by ODInfo.OID into g
select new OutputType
{
OID = g.Key,
TotalTons = g.AsQueryable().Sum(expr)
};
}
var query = myQuery(DataContext.CIMS_TRUCKS, ODInfo => ODInfo.TONS2009);
I haven't tried this, but did something similar here .
UPDATE
If you really need to translate input strings (like "2009") to expressions, it's still possible:
string year = "2009";
Type ODInfoType = typeof(ODINFOTYPE); // substitute with the type of ODInfo
ParameterExpression pe = ParameterExpression.Parameter(ODInfoType, "ODInfo");
MemberInfo mi = ODInfoType.GetProperty("TONS" + year);
MemberExpression me = Expression.MakeMemberAccess(pe, mi);
var expr = Expression.Lambda<Func<ODINFOTYPE, decimal>>(me, pe);
Remember, this is a patch for the extremely evil structure of your database.
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The best solution is to break this down into multiple queries that you can compose for the final query:
int year = 2009; // get summ of TONS2009 column
var odInfos =
year == 2009 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2009 })
year == 2010 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2010 })
year == 2011 ? DataContext.CIMS_TRUCKS.Select(x => new { x.OID, TONS = x.TONS2011 })
: null;
var query = from ODInfo in odInfos
where pLocationIDs.Contains(ODInfo.OID)
group ODInfo by ODInfo.OID into g
select new
{
OID = g.Key,
TotalTons = g.Sum(ODInfo => ODInfo.TONS)
};
This will specialize in three possible runtime queries, thus providing the best performance. It's better than case-switch.
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Try this way:
IEnumerable<OutputType> myQuery(IEnumerable<InputType> data, Func<InputType,decimal> func)
{
return from ODInfo in data
where pLocationIDs.Contains(ODInfo.OID)
group ODInfo by ODInfo.OID into g
select new OutputType
{
OID = g.Key,
TotalTons = g.Sum(func)
};
}
var query = myQuery(DataContext.CIMS_TRUCKS, ODInfo => ODInfo.TONS2009);
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Using DynamicLinq , which also works with EF:
int year = 2009; // get summ of TONS2009 column
var query = from ODInfo in DataContext.CIMS_TRUCKS
where pLocationIDs.Contains(ODInfo.OID)
group ODInfo by ODInfo.OID into g
select g;
var projectedGroups = query.Select("new (Key as OID, Sum(TONS" + year + ") as TotalTons)");
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