I need to get last record from db. I'm using sqlalchemy.
At the moment, I'm doing like that:
obj = ObjectRes.query.all()
return str(obj[-1].id)
But it's too heavy query. How can I get last record better?
Take a look at Query.first(). If you specify a sort on the right column, the first will be your last. An example could look like this:
obj = session.query(ObjectRes).order_by(ObjectRes.id.desc()).first()
Sometimes it is difficult to reformulate simple things:
SELECT * FROM ObjectRes WHERE id IN (SELECT MAX(id) FROM ObjectRes)
but this worked for me:
session.query(ObjectRes).filter(ObjectRes.id == session.query(func.max(ObjectRes.id)))
Don't forget to disable existing ordering if needed
In my case I have dynamic ordered relationships:
class Match:
...
records = relationship("Record", backref="match", lazy="dynamic", order_by="Record.id")
And when I tried accepted answer I got first record, not the last, cause ORDER BY were applied twice and spoiled the results.
According to documentation:
All existing ORDER BY settings can be suppressed by passing None
So the solution will be:
match = db_session.query(Match).first()
last_record = match.records.order_by(None).order_by(Record.id.desc()).first()
This answer modifies the others to allow for cases where you don't know what the primary key is called.
from sqlalchemy.inspection import inspect
# ...
def last_row(Table: type, *, session): # -> Table
primary_key = inspect(Table).primary_key[0].name # must be an arithmetic type
primary_key_row = getattr(Table, primary_key)
# get first, sorted by negative ID (primary key)
return session.query(Table).order_by(-primary_key_row).first()
Related
I have a MySQL DB with a table that has a SET type column with the following definition:
CREATE TABLE t (
col SET('V','A','L','U','E')
)
I would like to write a SELECT query that returns all the rows where col equals to ('A','L','E')
This can be done by writing the following query:
SELECT * FROM t WHERE c = 'A,L,E'
The query that i would like to write is one that will return the same result also for an non ordered input like 'L','A','E'
I couldn't find an elegant way to do so and couldn't find anything that can help me in the official documentation
You can fix nacho's suggestion using the following:
WHERE floor(pow(2,FIND_IN_SET('A',c)-1))+
floor(pow(2,FIND_IN_SET('L',c)-1))+
floor(pow(2,FIND_IN_SET('E',c)-1))=c
This is by no means an "elegant solution"... I would rather use a simpler one if possible.
FIND_IN_SET provides the position in the enum, so we have to raise 2 by this number to get the internal representation of the SET value.
The floor() function is used to keep the expression 0 when find_in_set returns 0.
Note that you still have the risk of false positives when checking against illegal SET values (e.g. looking for 'A','L','E' and 'X' will return true)
You need to use the FIND IN SET
SELECT *
FROM t
WHERE FIND_IN_SET('A',c)>0 AND FIND_IN_SET('L',c)>0 AND FIND_IN_SET('E',c)>0
I don´t know if this will work but you can also try:
SELECT *
FROM t
WHERE FIND_IN_SET('A,L,E',c)>0
Another possible approach is to check each item separately + check that the sizes of the groups match (the assumption is that the searched set has no repetitions):
SELECT *
FROM t
WHERE FIND_IN_SET('A',c)>0 AND FIND_IN_SET('L',c)>0 AND FIND_IN_SET('E',c)>0 AND BIT_COUNT(c) = 3
I want to specify the return values for a specific update in sqlalchemy.
The documentation of the underlying update statement (sqlalchemy.sql.expression.update) says it accepts a "returning" argument and the docs for the query object state that query.update() accepts a dictionary "update_args" which will be passed as the arguments to the query statement.
Therefore my code looks like this:
session.query(
ItemClass
).update(
{ItemClass.value: value_a},
synchronize_session='fetch',
update_args={
'returning': (ItemClass.id,)
}
)
However, this does not seem to work. It just returns the regular integer.
My question is now: Am I doing something wrong or is this simply not possible with a query object and I need to manually construct statements or write raw sql?
The full solution that worked for me was to use the SQLAlchemy table object directly.
You can get that table object and the columns from your model easily by doing
table = Model.__table__
columns = table.columns
Then with this table object, I can replicate what you did in the question:
from your_settings import db
update_statement = table.update().returning(table.id)\
.where(columns.column_name=value_one)\
.values(column_name='New column name')
result = db.session.execute(update_statement)
tuple_of_results = result.fetchall()
db.session.commit()
The tuple_of_results variable would contain a tuple of the results.
Note that you would have to run db.session.commit() in order to persist the changes to the database as you it is currently running within a transaction.
You could perform an update based on the current value of a column by doing something like:
update_statement = table.update().returning(table.id)\
.where(columns.column_name=value_one)\
.values(like_count=table_columns.like_count+1)
This would increment our numeric like_count column by one.
Hope this was helpful.
Here's a snippet from the SQLAlchemy documentation:
# UPDATE..RETURNING
result = table.update().returning(table.c.col1, table.c.col2).\
where(table.c.name=='foo').values(name='bar')
print result.fetchall()
I am using DBIx::Class and I would like to only update one row in my table. Currently this is how I do it:
my $session = my_app->model("DB::Session")->find(1);
$session->update({done_yn=>'y',end_time=>\'NOW()'});
It works, but the problem is that when it does find to find the row, it does this whole query:
SELECT me.id, me.project_id, me.user_id, me.start_time, me.end_time, me.notes, me.done_yn FROM sessions me WHERE ( me.id = ? ): '8'
Which seems a bit much when all I want to do is update a row. Is there anyway to update a row without having to pull the whole row out of the database first? Something like this is what I am looking for:
my_app->model("DB::Session")->update({done_yn=>'y',end_time=>\'NOW()'},{id=>$id});
Where $id is the WHERE id=? part of the query. Does anyone know how to do this? Thanks!
You can run update on a restricted resultset which only matches this single row:
my_app->model("DB::Session")->search_rs({ id=> 1 })->update({done_yn=>'y',end_time=>\'NOW()'});
I suggest you use a DateTime->now object instead of literal SQL for updating the end_time column because it uses the apps servers date and time instead of the database servers and makes your schema more compatible with different RDBMSes.
Do you have a check if the row was found to prevent an error in case it wasn't?
You might want to use update_or_create instead.
You could use the "columns" attribute:
my $session = my_app->model("DB::Session")->find(1, {columns => "id"});
I'm using LINQ to Entities to display paged results. But I'm having issues with the combination of Skip(), Take() and OrderBy() calls.
Everything works fine, except that OrderBy() is assigned too late. It's executed after result set has been cut down by Skip() and Take().
So each page of results has items in order. But ordering is done on a page handful of data instead of ordering of the whole set and then limiting those records with Skip() and Take().
How do I set precedence with these statements?
My example (simplified)
var query = ctx.EntitySet.Where(/* filter */).OrderByDescending(e => e.ChangedDate);
int total = query.Count();
var result = query.Skip(n).Take(x).ToList();
One possible (but a bad) solution
One possible solution would be to apply clustered index to order by column, but this column changes frequently, which would slow database performance on inserts and updates. And I really don't want to do that.
EDIT
I ran ToTraceString() on my query where we can actually see when order by is applied to the result set. Unfortunately at the end. :(
SELECT
-- columns
FROM (SELECT
-- columns
FROM (SELECT -- columns
FROM ( SELECT
-- columns
FROM table1 AS Extent1
WHERE EXISTS (SELECT
-- single constant column
FROM table2 AS Extent2
WHERE (Extent1.ID = Extent2.ID) AND (Extent2.userId = :p__linq__4)
)
) AS Project2
limit 0,10 ) AS Limit1
LEFT OUTER JOIN (SELECT
-- columns
FROM table2 AS Extent3 ) AS Project3 ON Limit1.ID = Project3.ID
UNION ALL
SELECT
-- columns
FROM (SELECT -- columns
FROM ( SELECT
-- columns
FROM table1 AS Extent4
WHERE EXISTS (SELECT
-- single constant column
FROM table2 AS Extent5
WHERE (Extent4.ID = Extent5.ID) AND (Extent5.userId = :p__linq__4)
)
) AS Project6
limit 0,10 ) AS Limit2
INNER JOIN table3 AS Extent6 ON Limit2.ID = Extent6.ID) AS UnionAll1
ORDER BY UnionAll1.ChangedDate DESC, UnionAll1.ID ASC, UnionAll1.C1 ASC
My workaround solution
I've managed to workaround this problem. Don't get me wrong here. I haven't solved precedence issue as of yet, but I've mitigated it.
What I did?
This is the code I've used until I get an answer from Devart. If they won't be able to overcome this issue I'll have to use this code in the end.
// get ordered list of IDs
List<int> ids = ctx.MyEntitySet
.Include(/* Related entity set that is needed in where clause */)
.Where(/* filter */)
.OrderByDescending(e => e.ChangedDate)
.Select(e => e.Id)
.ToList();
// get total count
int total = ids.Count;
if (total > 0)
{
// get a single page of results
List<MyEntity> result = ctx.MyEntitySet
.Include(/* related entity set (as described above) */)
.Include(/* additional entity set that's neede in end results */)
.Where(string.Format("it.Id in {{{0}}}", string.Join(",", ids.ConvertAll(id => id.ToString()).Skip(pageSize * currentPageIndex).Take(pageSize).ToArray())))
.OrderByDescending(e => e.ChangedOn)
.ToList();
}
First of all I'm getting ordered IDs of my entities. Getting only IDs is well performant even with larger set of data. MySql query is quite simple and performs really well. In the second part I partition these IDs and use them to get actual entity instances.
Thinking of it, this should perform even better than the way I was doing it at the beginning (as described in my question), because getting total count is much much quicker due to simplified query. The second part is practically very very similar, except that my entities are returned rather by their IDs instead of partitioned using Skip and Take...
Hopefully someone may find this solution helpful.
I haven't worked directly with Linq to Entities, but it should have a way to hook specific stored procedures into certain locations when needed. (Linq to SQL did.) If so, you could turn this query into a stored procedure, doing exacly what is required, and doing it efficiently.
Assuming from you comment the persisting the values in a List is not acceptable:
There's no way to completely minimize the iterations, as you intended (and as I would have tried too, living in hope). Cutting the iterations down by one would be nice. Is it possible to just get the Count once and cache/session it? Then you could:
int total = ctx.EntitySet.Count; // Hopefully you can not repeat doing this.
var result = ctx.EntitySet.Where(/* filter */).OrderBy(/* expression */).Skip(n).Take(x).ToList();
Hopefully you can cache the Count somehow, or avoid needing it every time. Even if you can't, this is the best you can do.
Could you please create a sample illusrating the problem and send it to us (support * devart * com, subject "EF: Skip, Take, OrderBy")?
Hope we will be able to help you.
You can also contact us using our forums or contact form.
Are you absolutely certain the ordering is off? What does the SQL look like?
Can you reorder your code as follows and post the output?
// Redefine your queries.
var query = ctx.EntitySet.Where(/* filter */).OrderBy(e => e.ChangedDate);
var skipped = query.Skip(n).Take(x);
// let's look at the SQL, shall we?
var querySQL = query.ToTraceString();
var skippedSQL = skipped.ToTraceString();
// actual execution of the queries...
int total = query.Count();
var result = skipped.ToList();
Edit:
I'm absolutely certain. You can check my "edit" to see trace result of my query with skipped trace result that is imperative in this case. Count is not really important.
Yeah, I see it. Wow, that's a stumper. Might even be an outright bug. I note you're not using SQL Server... what DB are you using? Looks like it might be MySQl.
One way:
var query = ctx.EntitySet.Where(/* filter */).OrderBy(/* expression */).ToList();
int total = query.Count;
var result = query.Skip(n).Take(x).ToList();
Convert it to a List before skipping. It's not too efficient, mind you...
I have a few rows of data pulled into business objects via linq-to-sql from large tables.
Now I want to get a few rows that don't match to test my comparison functions.
Using what I thought would work I get a NotSupportedException:
Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator.
Here's the code:
//This table has a 2 field primary key, the other has a single
var AllNonMatches = from c in dc.Acaps
where !Matches.Rows.Any((row) => row.Key.Key == c.AppId & row.Key.Value == c.SeqNbr)
select c;
foreach (var item in AllNonMatches.Take(100)) //Exception here
{}
The table has a compound primary key: AppId and SeqNbr.
The Matches.Rows is defined as a dictionary of keyvaluepair(appid,seqnbr).
and the local sequence it is referring to appears to be the local dictionary.
Could you provide more information on the structure and the name(s) of the table(s) plz?
Not sure what you're trying to do...
edit:
Ok.. I think I get it now...
It appears you can't merge/join local tables (dictionary) with a SQL table.
If you can, I'm afraid I don't know how to do it.
The simplest solution I can think of is to put those results in a table ("Match" for instance) with foreign keys related to your table "Acaps" and then use linq-to-sql, like:
var AllNonMatches = dc.Acaps.Where(p=>p.Matchs==null).Take(100).ToList();
Sorry I couldn't come up with any better =(
What about this:
var AllNonMatches = from c in dc.Acaps
where !(Matches.Rows.ContainsKey(c.AppId) && Matches.Rows.ContainsValue(c.SeqNbr))
select c;
That will work fine. I have also used a bitwise AND operator (&&) - I think thats the right term to help improve performance over the standard AND operator.