I have a very simple question but cannot find a simple answer.
I have NHibernate executing a raw SQL query, which unfortunately is loaded from the DB (yes, we store SQL query in the DB, not the best design but bear with me). This means that basically I have no idea how many columns the query will return.
What I know is that if the query is a single column then it is what I need and if it is more than one column then I need the first one, easy enough.
In my code I have basically 3 options:
session.CreateSQLQuery(SQLString).List();
session.CreateSQLQuery(SQLString).List<object>();
session.CreateSQLQuery(SQLString).List<object[]>();
Problem is that the List() will return either a List<int> (or the appropriate type, but int should be in my case) if the query returns a single column, or a List<object[]>() if I have multiple columns. Same goes for List<object> except that it will not return a List<int> in this case.
Of course trying to always cast to object[] (3rd option) and get the first element does not work, since nHibernate returns an int and that cannot be cast to object[].
Is there a way to force nHibernate to always return an object[] even in the case of a single column? If not, is there an easy way to check the number of columns in the result and act accordingly?
Thanks!
You should take a look at the differant result transformers NHibernate provides out of the box (you can write your own as well of course).
What might fit in your case would be the NHibernate.Transform.ToListResultTransformer. AFAIK, it transforms your result to an IList, where each entry contains an IList again.
So:
IList items = session
.CreateSQLQuery(query)
.SetResultTransformer(new NHibernate.Transform.ToListResultTransformer())
.List()
if(((Ilist)items[0]).Count == 1)
// just one columns
else
// more columns or zero
Related
I am sure this is easy (or that I am completely wrong), but all I want to do is get the single values against a LINQ query on a datatable and not use a for.. each loop to access them.
Scenario..
I have a datatable containing a number of rows. I run my query looking for a particular match on "EP" like such..
Dim qryB = From rw In oDT_Cables
Where rw("EP").Equals(br.Position)
Select ld = rw("LOAD"), tl = rw("TOT-LEN")
Now..
I know that I have only one result (due to checking upon insertion), so how do I simply access the items ld and tl without a for each loop??
I have tried:
qryb.elementat(1) error
qryb.elementat(ld) Not possible
qryb.tl not possible
Does anyone know how I can access tl and ld from the query please? It is much appreciated.
Brett
If you know for a fact that a LINQ query produces a single result then you call Single.
Dim result = qryB.Single()
If there may be no result but will never be more than one then you call SingleOrDefault and the result may be Nothing. If there will be at least one result but may be more and you want just the first, you call First. If there may be zero, one or more and you want the first if there is one then you call FirstOrDefault and the result may be Nothing.
Note that some LINQ providers don't support both options, e.g. LINQ to Entities (the LINQ provider for Entity Framework) supports First and FirstOrDefault but not Single or SingleOrDefault (unless it's changed recently) while, if I'm not mistaken, LINQ to SQL supports Single and SingleOrDefault but not First or FirstOrDefault.
Note that, if you were going to use ElementAt, you would have to specify 0 as the index. ElementAt works pretty much like indexing a array or other IList, so the first item is at index 0. You'd only use ElementAt for something in the middle of a query result though, given that you have Single, First and Last methods and their nullable equivalents.
I have a table, CityComplete, with columns "USPSCITY", "STATE" and "ZIPCODE"
I have an existing SQLQuery that distinctly selects USPSCITY and STATE based on a fuzzy search. However, when I call list() I get an exception:
19. ResultSet.getString(ZIPCODE)
java.sql.SQLException: Column 'ZIPCODE' not found.
The SQLQuery's entity is set to the CityComplete object.
Is there any way to have Hibernate not try and get ZIPCODE, or have it be part of the result set in some way?
What's the best way to resolve this, other than using Criteria, setting a virtual column with null data, or just getting a full result set and handling distinction on the code side?
When you perform a regular Hibernate query (HQL or Criteria), Hibernate will try to map the resultset based on the properties of the entities which you specified in the mapping. If you are not bringing the "promised" data from the database, you'll need to handle the mapping by yourself, using a ResultTransformer. In this case, you'd still use your CityComplete, but they will be without a ZIPCODE. Unfortunately, there's not much documentation about how to use a ResultTransformer, but you can take a look at the Hibernate's JavaDoc and see if there's one that you could use. If not, you'd need to implement your own:
http://docs.jboss.org/hibernate/core/3.6/javadocs/org/hibernate/transform/ResultTransformer.html
MyModel.objects.filter(field__icontains=value) returns all the rows whose field contains value. How to do the opposite? Namely, construct a queryset that returns all the rows whose field is contained in value?
Preferably without using custom SQL (ie only using the ORM) or without using backend-dependent SQL.
field__icontains and similar are coded right into the ORM. The other version simple doesn't exist.
You could use the where param described under the reference for QuerySet.
In this case, you would use something like:
MyModel.objects.extra(where=["%s LIKE CONCAT('%%',field,'%%')"], params=[value])
Of course, do keep in mind that there is no standard method of concatenation across DMBS. So as far as I know, there is no way to satisfy your requirement of avoiding backend-dependent SQL.
If you're okay with working with a list of dictionaries rather than a queryset, you could always do this instead:
qs = MyModel.objects.all().values()
matches = [r for r in qs if value in r[field]]
although this is of course not ideal for huge data sets.
I have Django models setup in the following manner:
model A has a one-to-many relationship to model B
each record in A has between 3,000 to 15,000 records in B
What is the best way to construct a query that will retrieve the newest (greatest pk) record in B that corresponds to a record in A for each record in A? Is this something that I must use SQL for in lieu of the Django ORM?
Create a helper function for safely extracting the 'top' item from any queryset. I use this all over the place in my own Django apps.
def top_or_none(queryset):
"""Safely pulls off the top element in a queryset"""
# Extracts a single element collection w/ top item
result = queryset[0:1]
# Return that element or None if there weren't any matches
return result[0] if result else None
This uses a bit of a trick w/ the slice operator to add a limit clause onto your SQL.
Now use this function anywhere you need to get the 'top' item of a query set. In this case, you want to get the top B item for a given A where the B's are sorted by descending pk, as such:
latest = top_or_none(B.objects.filter(a=my_a).order_by('-pk'))
There's also the recently added 'Max' function in Django Aggregation which could help you get the max pk, but I don't like that solution in this case since it adds complexity.
P.S. I don't really like relying on the 'pk' field for this type of query as some RDBMSs don't guarantee that sequential pks is the same as logical creation order. If I have a table that I know I will need to query in this fashion, I usually have my own 'creation' datetime column that I can use to order by instead of pk.
Edit based on comment:
If you'd rather use queryset[0], you can modify the 'top_or_none' function thusly:
def top_or_none(queryset):
"""Safely pulls off the top element in a queryset"""
try:
return queryset[0]
except IndexError:
return None
I didn't propose this initially because I was under the impression that queryset[0] would pull back the entire result set, then take the 0th item. Apparently Django adds a 'LIMIT 1' in this scenario too, so it's a safe alternative to my slicing version.
Edit 2
Of course you can also take advantage of Django's related manager construct here and build the queryset through your 'A' object, depending on your preference:
latest = top_or_none(my_a.b_set.order_by('-pk'))
I don't think Django ORM can do this (but I've been pleasantly surprised before...). If there's a reasonable number of A record (or if you're paging), I'd just add a method to A model that would return this 'newest' B record. If you want to get a lot of A records, each with it's own newest B, I'd drop to SQL.
remeber that no matter which route you take, you'll need a suitable composite index on B table, maybe adding an order_by=('a_fk','-id') to the Meta subclass
I'm trying to reproduce the HqlQuery style 'select new ObjectToProjectOut' functionality. i.e. take a list of columns returned from a query and return as a list of ObjectToProjectOut types that are instantiated using a Constructor with as many parameters as the columns in the query.
This is in effect what 'select new ObjectToProjectOut' achieves in Hql.... but clearly that's not available in SqlQuery. I think I need to set a result transform and use either PassThroughResultTransformer, DistinctRootEntityResultTransformer etc to get it to work.
Anyone know what I should use ?
ok.... after looking at the NHibernate code it seems that I was looking for AliasToBeanConstructorResultTransformer.... of course!
However I may have found an nHibernate bug. If you have the same column name returned twice from two different tables (market.name and account.name, say) then by the time nHibernate returns the array from the db to the transformer, the first occurance of 'Name' will be used for both. Nasty.
Work around is to uniquely alias. With Hql, the sql generated is heavily aliased, so this is only a bug with SqlQuery.
Grrrr. Today must be my day, also found another nHibernate bug/issue I've posted to StackOverflow for comment.
You could use the AddEntity method to fill entities from a SQL query.
Here are two examples from the NHibernate docs:
sess.CreateSQLQuery("SELECT * FROM CATS")
.AddEntity(typeof(Cat));
sess.CreateSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS")
.AddEntity(typeof(Cat));