I need help with Linq Contains method. Here's the code below.
This code does work but outputs an empty sets.
if (request.OperatorId != null && request.OperatorId != Guid.Empty)
{
var checkOperator = _context.Operators.Include(a => a.OperatorLevel).Include(a => a.City).Include("City.StateRegion.Country").FirstOrDefault(a => a.Id == request.OperatorId);
List<String> Cities = new List<String>();
if (checkOperator.OperatorLevel.Name == "City")
{
Cities = await _context.Cities
.Where(a => (checkOperator.CityId) == (a.Id))
.Select(a => a.Code)
.ToListAsync();
}
else if (checkOperator.OperatorLevel.Name == "Regional")
{
Cities = await _context.Cities
.Where(a => checkOperator.City.StateRegionId == a.StateRegionId)
.Select(a => a.Code)
.ToListAsync();
}
else if (checkOperator.OperatorLevel.Name == "National")
{
List<Guid> StateRegion = await _context.StateRegions
.Where(a => checkOperator.City.StateRegion.CountryId == a.CountryId)
.Select(a => a.Id)
.ToListAsync();
Cities = await _context.Cities
.Where(a => StateRegion.Contains(a.StateRegionId))
.Select(a => a.Code)
.ToListAsync();
}
var nullableStrings = Cities.Cast<String?>().ToList();
query = query.Where(a => nullableStrings.Contains(a.Code));
}
I need to compare nullableStrings to a.Code which is something like this, but does not work.
query = query.Where(a => a.Code.Contains(nullableStrings));
Error : Argument 1: cannot convert from 'System.Collections.Generic.List' to 'char'
I need a method that would replace
query = query.Where(a => nullableStrings.Contains(a.Code));
to something like
query = query.Where(a => a.Code.Contains(nullableStrings));
but that works. A help would be appreciated. Thanks.
CodePudding user response：
Looking at the code, my guess is the requirement is to get a list of operators depending on the current (check) operator's level. I suspect the issue you are encountering is that some cities may not have a code. You then want to apply all found codes to another query that you are building up.
My guess is that the crux of the problem is that some cities might not have a code, hence the concern for null-able strings, while others might have multiple codes hacked into a single-code intended field. The solution there would typically be to remove any null values
Firstly, this line:
var checkOperator = _context.Operators.Include(a => a.OperatorLevel).Include(a => a.City).Include("City.StateRegion.Country").FirstOrDefault(a => a.Id == request.OperatorId);
can be simplified to:
var checkOperator = _context.Operators
.Select(a => new
{
Level = a.OperatorLevel.Name,
CityId = a.City.Id,
CityCode = a.City.Code,
StateRegionId = a.City.StateRegion.Id,
CountryId = a.City.StateRegion.Country.Id
}).FirstOrDefault(a => a.Id == request.OperatorId);
This builds a faster query, rather than fetching an entire operator object graph, just select the fields from the object graph that we need.
Now to handle the operator level. Here I don't recommend trying to force every scenario into a single pattern. The goal is just to apply a filter to the built query, so have the scenarios do just that:
select (checkOperator.Level)
{
case "City":
query = query.Where(a => a.Code == checkOperator.CityCode);
break;
case "Regional":
var cityCodes = await _context.Cities
.Where(a => a.Code != null && a.StateRegion.Id == checkOperator.StateRegionId)
.Select(a => a.Code)
.ToListAsync();
query = query.Where(a => cityCodes.Contains(a.Code));
break;
case "Country":
var cityCodes = await _context.Cities
.Where(a => a.Code != null && a.StateRegion.Country.Id == checkOperator.CountryId)
.Select(a => a.Code)
.ToListAsync();
query = query.Where(a => cityCodes.Contains(a.Code));
break;
}
Now based on the comments it sounds like your data with cities and codes is breaking proper normalization where Code was intended as a 1-to-1 but later hacked to handle one city having multiple codes, so multiple values were concatenated with hyphens. (I.e. ABC-DEF) If this represents 2 Codes for the city then you will need to handle this..
private List<string> splitCityCodes(List<string> cityCodes)
{
if (cityCodes == null) throw NullReferenceException(nameof(cityCodes));
if (!cityCodes.Any()) throw new ArgumentException("At least one city code is expected.");
var multiCodes = cityCodes.Where(x => x.Contains("-")).ToList();
if (!multiCodes.Any())
return cityCodes;
var results = new List<string>(cityCodes);
results.RemoveRange(multiCodes);
foreach(var multiCode in multiCodes)
{
var codes = multiCode.Split("-");
results.AddRange(codes);
}
return results.Distinct();
}
That can probably be optimized, but the gist is to take the city codes, look for hyphenated values and split them up, then return a distinct list to remove any duplicates.
List<string> cityCodes = new List<string>();
select (checkOperator.Level)
{
case "City":
cityCodes = splitCityCodes(new []{checkOperator.CityCode}.ToList());
if(cityCodes.Count == 1)
query = query.Where(a => a.Code == cityCodes[0]);
else
query = query.Where(a => cityCodes.Contains(a.Code));
break;
case "Regional":
cityCodes = await _context.Cities
.Where(a => a.Code != null && a.StateRegion.Id == checkOperator.StateRegionId)
.Select(a => a.Code)
.ToListAsync();
cityCodes = splitCityCodes(cityCodes);
query = query.Where(a => cityCodes.Contains(a.Code));
break;
case "Country":
cityCodes = await _context.Cities
.Where(a => a.Code != null && a.StateRegion.Country.Id == checkOperator.CountryId)
.Select(a => a.Code)
.ToListAsync();
cityCodes = splitCityCodes(cityCodes);
query = query.Where(a => cityCodes.Contains(a.Code));
break;
}
... and I suspect that would about do it for handling the possibility of a city code containing multiple values.
CodePudding user response：
If your search argument is in the form "ABC-DEF" and you want that to match "ABC" OR "DEF" then it can be done, but it is not clear from your data setup how that scenario comes about.
Lets assume these codes are airport codes, and that a city that has multiple airports has the City.Code as a hyphenated list of the Airport codes, then if the checkOperator is in Australia, and their OperatorLevel is "National" then this might build the following nullableStrings:
var Cities = new List<string> {
"PER",
"ADE",
"DRW",
"MEL-AVV",
"SYD",
"BNE",
"OOL",
"HBA"
};
If then your query is a listing of AirPorts and you want to search the airports by these codes, specifically to match both "MEL" and "AVV" then you can use syntax like this
var nullableStrings = Cities.Cast<String?>().ToList();
query = query.Where(ap => nullableStrings.Any(n => n.Contains(ap.Code)));
But if you intend this to be translated to SQL via LINQ to Entities (so be executed server-side) then we can make this query more efficient buy normalizing the search args so we can do an exact match lookup:
var nullableStrings = Cities.Where(x => !String.IsNullOrWhiteSpace (x))
.SelectMany(x => x.Split('-'))
.Cast<String?>()
.ToList();
query = query.Where(ap => nullableStrings.Contains(ap.Code));
As this routine is called as part of a larger set and your checkOperator goes out of scope, you should try to reduce the fields that you retrieve from the database to the specific set that this query needs through a projection
Using
.Select()to project out specific fields can help improve the overall efficiency of the database, not just each individual query. If the additional fields are minimal or natural surrogate keys, and your projections are common to other query scenarios then they can make good candidates for specific index optimizations.
Instead of loading SELECT * from all these table in this include list:
var checkOperator = _context.Operators.Include(a => a.OperatorLevel)
.Include(a => a.City.StateRegion.Country)
.FirstOrDefault(a => a.Id == request.OperatorId);
So instead of all the fields from OperatorLevel, City, StateRegion, Country we can load just the fields that our logic needs:
var checkOperator = _context.Operators.Where(o => o.Id == request.OperatorId)
.Select(o => new {
OperatorLevelName = o.OperatorLevel.Name,
o.CityId,
o.City.StateRegionId,
o.City.StateRegion.CountryId
})
.FirstOrDefault();
So many of the EF has poor performance opinions out there stem from a lot of poorly defined examples that proliferate the web. Eagerly loading is the same as executing
SELECT * FROM ...for simple tables it's only a bandwidth and memory waste, but for complex tables that have computed columns or custom expressions there can be significant server CPU costs.It cannot be overstated the improvements that you can experience if you use projections to expose only the specific sub-set of the data that you need, especially if you will not be attempting to modify the results of the query.
Be a good corporate citizen, only take what you need!
So lets put this back into your logic:
if (request.OperatorId != null && request.OperatorId != Guid.Empty)
{
var checkOperator = _context.Operators.Where(o => o.Id == request.OperatorId)
.Select(o => new {
OperatorLevelName = o.OperatorLevel.Name,
o.CityId,
o.City.StateRegionId,
o.City.StateRegion.CountryId
})
.FirstOrDefault();
IQueryable<City> cityQuery = null;
if (checkOperator.OperatorLevelName == "City")
cityQuery = _context.Cities
.Where(a => checkOperator.CityId == a.Id);
else if (checkOperator.OperatorLevelName == "Regional")
cityQuery = _context. Cities
.Where(a => checkOperator.StateRegionId == a.StateRegionId);
else if (checkOperator.OperatorLevelName == "National")
cityQuery = _context. Cities
.Where(c => c.StateRegion.CountryId == checkOperator.CountryId);
// TODO: is there any default filter when operator level is something else?
if (cityQuery != null)
{
var nullableStrings = cityQuery.Select(a => a.Code)
.ToList()
.Where(x => !String.IsNullOrWhiteSpace(x))
.SelectMany(x => x.Split('-'))
.Cast<String?>()
.ToList();
query = query.Where(ap => nullableStrings.Contains(ap.Code));
}
}
If you don't want or need to normalize the strings, then you can defer this whole expression without realizing the city query at all:
// No nullable string, but we can still remove missing Codes
cityQuery = cityQuery.Where(c => c.Code != null);
query = query.Where(ap => cityQuery.Any(c => c.Code.Contains(ap.Code)));