I'm trying to figure out how to modify this to match on 2:
var result = _context.FirstOrDefault(c => c.CarId == carId);
I'm not sure how to tack this on. I just want to base it on c.CarId == carId && c.UserId == userId
where carId and userId are incoming params to my method that this LINQ statement resides in. I want to keep this as a lambda expression syntax.
Just do it exactly as you've written it:
var result = _context.FirstOrDefault(c => c.CarId == carId && c.UserId == userId);
There's nothing wrong with that. The lambda expression isn't restricted to compare a single property.
If you want to learn about LINQ in more detail, I'd start with LINQ to Objects, which is simpler to understand and predict. There are various tutorials around for it, and I have a blog series called Edulinq which examines each operator in detail.
Related
In my app I have some queries that use the same repeated logic:
var someThings = context.table1
.where(SomeLogic)
.ToList();
With EF Core 2.1 I could encapsulate this logic in a layer with all these expressions:
public static Expression<Func<MyObject, bool>> SomeLogic =>
myObject => myObject.CreationDate.Date == DateTime.Now.Date
&& (myObject.Whatever.HasValue || myObject.MoreWhatever);
Now I discovered this was being evaluated in memory, and that's bad.
If I do something like:
var someThings = context.table1
.where(myObject =>
myObject.CreationDate.Date == DateTime.Now.Date
&& (myObject.Whatever.HasValue || myObject.MoreWhatever))
.ToList();
then the query is evaluated in the DB, but I am putting some logic in the wrong layer.
I tried to subsitute Expression with a function or any other tool, but I don't find a way to do it.
Is there a way to encapsulate the logic of a query in a layer as I was doing before, but preserving EF rules so that this query can still be evaluated in the DB?
Thanks.
Why you need a "real" expression and not just a Lambda is explained in this answer. The created Expression can be created anywhere and passed as a parameter to the function that executes the query.
This answer should guide the way you need to go. You only have to replace the two dummy expressions with the whatever.hasvalue...stuff
var param = Expression.Parameter(typeof(MyObject), nameof(MyObject));
// myObject.CreationDate.Date == DateTime.Now.Date
Expression dateExpression = Expression.Equal(Expression.Constant(DateTime.Now),
Expression.PropertyOrField(param, "CreationDate"));
var dummyExpression1 = Expression.Equal(Expression.Constant(1), Expression.Constant(1));
var dummyExpression2 = Expression.Equal(Expression.Constant(1), Expression.Constant(1));
// && (myObject.Whatever.HasValue || myObject.MoreWhatever)
Expression orExpression = Expression.Or(dummyExpression1, dummyExpression2);
Expression allConditions = Expression.And(dateExpression, orExpression);
//myObject =>
Expression myExpression = Expression.Lambda<Func<MyObject, bool>>(allConditions, param);
var someThings = context.table1
.where(myExpression)
.ToList();
I had the most trouble with Expression.PropertyOrField. If you have nested structures you need to loop through the data structure and call Expression.PropertyOrField with the first parameter being the result from the previous call to Expression.PropertyOrField.
var sample = db.Database.OrderByDescending(x => x.RecordId).Select(y => y.RecordId).FirstOrDefault();
I don't know if my title is correct / right. Just want to ask why this query the select is in another ( )?. As for the example .Select(y => y.RecordId) unlike the query I use to be
var sample = (from s in db.Databse where s.RecordId == id select s) I know this is the same right?. Then what is the why it is in another parenthesis?. Anyone has an idea or can anyone explain it why?. Thanks a lot.
In your first example, you're using "regular" C# syntax to call a bunch of extension methods:
var sample = db.Database
.OrderByDescending(x => x.RecordId)
.Select(y => y.RecordId)
.FirstOrDefault();
(They happen to be extension methods here, but of course they don't have to be...)
You use lambda expressions to express how you want the ordering and projection to be performed, and the compiler converts those into expression trees (assuming this is EF or similar; it would be delegates for LINQ to Objects).
The second example is a query expression, although it doesn't actually match your first example. A query expression corresponding to your original query would be:
var sample = (from x in db.Database
orderby x.RecordId descending
select x.RecordId)
.FirstOrDefault();
Query expressions are very much syntactic sugar. The compiler effectively converts them into the first form, then compiles that. The range variable declared in the from clause (x in this case) is used as the parameter name for the lambda expression, so select x.RecordId becomes .Select(x => x.RecordId).
Things become a bit more complicated with joins and multiple from clauses, as then the compiler introduces transparent identifiers to allow you to work with all the range variables that are in scope, even though you've really only got a single parameter. For example, if you had:
var query = from person in people
from job in person.Jobs
order by person.Name
select new { Person = person, Job = job };
that would be translated into the equivalent of
var query = people.SelectMany(person => person.Jobs, (person, job) => new { person, job } )
.OrderBy(t => t.person.Name)
.Select(t => new { Person = t.person, Job = t.job });
Note how the compiler introduces an anonymous type to combine the person and job range variables into a single object, which is used later on.
Basically, query expression syntax makes LINQ easier to work with - but it's just a translation into other C# code, and is neatly wrapped up in a single section of the C# specification. (Section 7.16.2 of the C# 5 spec.)
See my Edulinq blog post on query expressions for more detail on the precise translation from query expressions to "regular" C#.
I am trying to access a user object in a collection with the id = to users101 and set this to another users.
Controller.MyObject.SingleOrDefault(x => x.Id == "user101") = OtherUser();
Thanks in advance.
You can't do it with one LINQ expression.
Usually LINQ extensions works on enumerables, if MyObject is a collection you first have to find the required item and then overwrite it with the new object (moreover SingleOrDefault() will simply return null if condition is not satisfied).
You should write something like this (exact code depends on what MyObject is):
var item = Controller.MyObject.SingleOrDefault(x => x.Id == "user101");
if (item != null)
Controller.MyObject[Controller.MyObject.IndexOf(item)] = new OtherUser();
Please note that if you do not really need the check performed by SingleOrDefault() you can simplify the code (and avoid the double search performed in SingleOrDefault() and IndexOf()).
If this is "performance critical" maybe it is better to write an ad-hoc implementation that does this task in one single pass.
Try it in two lines:
var objectWithId = Controller.MyObject.SingleOrDefault(x => x.Id == "user101");
(objectWithId as WhateverTypeOfObjectOtherUserIs) = OtherUser();
I suspect I'm missing something rather basic, yet I can't figure this one out.
I'm running a simple linq query -
var result = from UserLine u in context.Users
where u.PartitionKey == provider.Value && u.RowKey == id.Value
select u;
UserLine user = null;
try
{
user = result.FirstOrDefault();
}
For some reason this produces a TargetInvocationException with an inner exception of NullReferenceException.
This happens when the linq query produces no results, but I was under the impression that FirstOrDefault would return Default<T> rather than throw an exception?
I don't know if it matters, but the UserLine class inherits from Microsoft.WindowsAzure.StorageClient.TableServiceEntity
there are two possible reasons:
provider.Value
id.Value
Are you sure that theese nullables have value. You might want to check HasValue before
var result = from UserLine u in context.Users
where (provider.HasValue && u.PartitionKey == provider.Value)
&& (id.HasValue && u.RowKey == id.Value)
select u;
UserLine user = null;
try
{
user = result.FirstOrDefault();
}
I thought it produced a different error, but based on the situation in which the problem is occurring you might want to look to check if context.IgnoreResourceNotFoundException is set to false? If it is try setting it to true.
This property is a flag to indicate whether you want the storage library to throw and error when you use both PartitionKey and RowKey in a query and no result is found (it makes sense when you think about what the underlying REST API is doing, but it's a little confusing when you're using LINQ)
I figured it out - the problem occured when either id or provider had '/' in the value, which the id did. when I removed it the code ran fine
Understanding the Table Service Data Model has a section on 'Characters Disallowed in Key Fields' -
The following characters are not allowed in values for the
PartitionKey and RowKey properties:
The forward slash (/) character
The backslash () character
The number sign (#) character
The question mark (?) character
Here's some fun try putting the where query the other way around like this to see if it works (I heard a while ago it does!):
where (id.HasValue && u.RowKey == id.Value) && (provider.HasValue && u.PartitionKey == provider.Value)
Other than this you can now set IgnoreResourceNotFoundException = true in the TableServiceContext to receive null when an entity is not found instead of the error.
It's a crazy Azure storage thing.
Me again with a dumb question/scenario I need advice on.
I have the following that pulls back the contents of a column:
return getappropriateuserfield.tblAutoComplete
.Where(p => p.MemberId == memberid && p.ACItem == acitem)
.Select(p => p.ACColumn)
.Distinct()
.ToArray();
Depending upon this result, I'd like to then take the ACColumn result, go to tblPreferences, look down ColumnName, and if it matches an entry in there, pull back the Alias (present in tblPreferences)
So, for example we have tblAutoComplete:
MemberID ACItem ACColumn
1 2 UUF1
tblPreferences looks like
MemberID ColumnName Alias
1 UUF1 Category
If the user sticks in "2" as the ACItem, the first part result would be "UUF1" - the linq above does this.
How do I alter the linq so that the second part takes place, ie. takes the UUF1, looks in tblPreferences, checks out ColumnName, sees the result matches so the final result is the Alias, "Category"
Do I need to do this in 2 parts or can I do it as one query, potentially using a join?
Apologies for the thickness.
Looks like a join to me, which is probably most easily expressed with a query expression:
var query = from ac in foo.tblAutoComplete
where ac.MemberId == memberid && ac.ACItem == acitem
join pref in foo.tblPreferences.Where(x => x.MemberId == memberid)
on ac.ACColumn equals pref.ColumnName
select pref.Category;
Note that I've removed the Distinct() call here, which means you may get repeats. You can put Distinct() on the output, of course.
The result of will be an IQueryable<string> (assuming Category is a string). If you need more bits, you could use an anonymous type.
EDIT: I've edited the query so it's got an extra "where" clause when fetching the preferences to start with. That should be equivalent to adding MemberId to the join.