pony.orm.core.TransactionError: An attempt to mix objects belonging to different transactions
I know that this error is descriptive, I just can't figure out WHY its occurring. I have searched google, the docs, other SO posts, and found nothing.
Any ideas?
This error arises if somebody attempts to interlink two objects which belongs to identity maps of two different db sessions:
from pony.orm.examples.university1 import *
populate_database()
with db_session:
g = Group[101]
with db_session:
s = Student[4]
s.group = g # or g.students.add(s)
Each db session should work with objects retrieved within this db session only. If you need to store some information between db sessions, you can store object's id value instead of object itself. Also you can pickle objects in one db_session and then unpickle them in another db_session.
Related
I have been reading about MediatR and CQRS latelly and I saw many people saying that commands shouldn't return domain objects. They can return values but they're limited to returning erros values, failure/success information and the Id of the newly created entities.
My question is how to return this new objetct to the client if the command can return only the Id of the new entity.
1) Should I query the database again with this new Id? If so, isn't that bad that I making a new trip to the database to get an object that was in the memory a few seconds ago?
2) What's the correct way of returning the entities created by the commands?
I think the more important question is why you shouldn't return domain objects from commands. If the reason for that seems like a valid reason for you, you should look into alternatives such as executing a query right after the command to fetch the domain object.
If, however, returning the domain object from the command fits your needs and does not impose any direct problems, then why not just do it and keep things simple and straightforward?
New to Sequel and SQL in general, so bear with me. I'm using Sequel's many_through_many plugin and I retrieve resources that are indirectly associated with particular tasks through groups, via a groups_tasks join table and a groups_resources join table. Then when I query task.resource on a Task dataset I get resource objects in Ruby, like so:
>>[#<Resource #values={:id=>2, :group_id=>nil, :display_name=>"some_name"}>, #<Resource #values={:id=>3, :group_id=>nil, :display_name=>"some_other_name"}>]
Now, I want to be able to add a new instance variable, schedule to these resource objects and do work on it in Ruby. However, every time I query task.resources for each task, Sequel is bringing resources objects in to ruby as different resource objects each time (which makes sense), despite being the same record in the database:
>>
"T3"
#<Resource:0x007fd4ca0c6fd8>
#<Resource:0x007fd4ca0c6920>
#<Resource:0x007fd4ca0c60d8>
#<Resource:0x007fd4ca0c57a0>
"T1"
#<Resource:0x007fd4ca0a4c08>
#<Resource:0x007fd4ca097f58>
#<Resource:0x007fd4ca097b48>
"T2"
#<Resource:0x007fd4ca085ba0>
#<Resource:0x007fd4ca0850d8>
I had wanted to just put a setter in class Resource and do resource.schedule = Schedule.new, but since they're all different objects, each resource is going to have a ton of different schedules. What's the most straightforward way to manipulate these resource objects client side, but maintain their task associations that I query from the server?
If I am understanding your question correctly, you want to retrieve Resource objects and then manipulate some attribute named schedule. I am not very familiar with Sequel, but looking over the docs it seems to work similarly to ActiveRecord.
Set up your instance variable (I imagine using something like attr_accessor :schedule on the Resource class).
Store the records in a variable, you will be working with same instance each time, rather than the new instance Sequel returns.
I am new to redis and I am trying to figure out how redis can be used.
So please let me know if this is a right way to build an application.
I am building an application which has got only one data source. I am planning to run a job on nightly basis to get data into a file.
Now I have a front end application, that needs to render this data in different formats.
Example application use case
Download processed applications by a university on nightly basis.
Display how many applications got approved or rejected.
Display number of applications by state.
Let user search for an application by application id.
Instead of using postgres/mysql like relational database, I am thinking about using redis. I am planning to store data in following ways.
Application id -> Application details
State -> List of application ids
Approved -> List of application ids (By date ?)
Declined -> List of application ids (By date ?)
Is this correct way to store data into redis?
Also if someone queries for all applications in california for a certain date,
I will be able to pull application ids in one call but to get details for each application, do I need to make another request?
Word of caution:
Instead of using postgres/mysql like relational database, I am thinking about using redis.
Why? Redis is an amazing database, but don't use the right hammer for the wrong nail. Use Redis if you need real time performance at scale, but don't try make it replace an RDBMS if that's what you need.
Answer:
Fetching data efficiently from Redis to answer your queries depends on how you'll be storing it. Therefore, to determine the "correct" data model, you first need to define your queries. The data model you proposed is just a description of the data - it doesn't really say how you're planning to store it in Redis. Without more details about the queries, I would store the data as follows:
Store the application details in a Hash (e.g. app:<id>)
Store the application IDs in a per state in Set (e.g. apps:<state>)
Store the approved/rejected applications in two Sorted Sets, the id being the member and the date being the score
Also if someone queries for all applications in california for a certain date, I will be able to pull application ids in one call but to get details for each application, do I need to make another request?
Again, that depends on the data model but you can use Lua scripts to embed this logic and execute it in one call to the database.
First of all you can use a Hash to store structured Data. With Lists (ZSets) and Sets you can create indexes for an ordered or unordered access. (Depending on your requirements of course. Make a list of how you want to access your data).
It is possible to get all data as json of an index in one go with a simple redis script (example using an unordered set):
local bulkToTable = function(bulk)
local retTable = {};
for index = 1, #bulk, 2 do
local key = bulk[index];
local value = bulk[index+1];
retTable[key] = value;
end
return retTable;
end
local functionSet = redis.call("SMEMBERS", "app:functions")
local returnObj = {} ;
for index = 1, #functionSet, 1 do
returnObj[index] = bulkToTable(redis.call("HGETALL", "app:function:" .. functionSet[index]));
returnObj[index]["functionId"] = functionSet[index];
end
return cjson.encode(returnObj);
more information about redis scripts see here : http://www.redisgreen.net/blog/intro-to-lua-for-redis-programmers/
Out of the blue, i am getting this error when doing a number of updates using nhibernate.
Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [MyDomainObject]
there is no additional information in the error. Is there some recommended way to help identify the root issue or can someone give me a better explanation on what this error indicated or is a sympton around.
Some additional info
I looked at the object and all of the data looks fine, it has an ID, etc . .
Note this is running in a single call stack from an asp.net-mvc website so i wouldn't expect there to be any threading issues to worry about in terms of concurrency.
NHibernate has an object, let's call it theObject. theObject.Id has a value of 42. NHibernate notices that the object is dirty. The object's Id is different than the unsaved-value, which is zero (0) for integer primary keys. So NHibernate issues an update statement, but no rows are updated, which means that there is no row in the database for that type of object with an Id of 42. So the object has been deleted without NHibernate knowing about it. This could happen inside another transaction (e.g. you have threading issues) or if someone (or another application) deleted/altered the row using SQL directly against the database.
The other possibility is that your unsaved-value is wrong. e.g. You are using -1 to indicate an unsaved-entity, but your mapping has a unsaved-value of zero. This is unlikely as your application is generally working from the sounds of it. If the unsaved-value was wrong, you wouldn't have been able to save any entities to the database as NHibernate would have been issuing UPDATE statements when it should have been issuing INSERT.
It means that you have multiple transactions accessing the same data, thus producing concurrency issues. You should improve on your data access handling, you probably are updating data from multiple threads, syndicate the changed data into a queue first which handles all the access to the db.
An old post, but hopefully my info will help someone. I was getting a similar error but only when persisting associations, after I had added in a new object. The error was of the form:
NHibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) [My.Entity#0]
Note the zero on the end, which is my identifier property. It should not be trying to save with key zero as I was using identity specification in SQL Server (generator class=native). I had not changed my unsaved-value in my xml so I had no idea what the problem was; for some reason NHibernate was trying to do an update using key value as 0 instead of a save (and getting the next key identity) for my new object.
In the end I found the cause was that I was initialising Version number to 1 for the new object in my constructor! Even though my identifier property was zero, for some reason NHibernate was also looking for a version property of zero as well, to identify it as an unsaved transient instance. The book "NHibernate in Action" does actually mention this on page 120, but for some reason my objects were fine when persisting with version number of 1 normally, and only failing if saving a new object through an association.
So make sure you do not set your Version value (leave as zero or null).
You say that your data is ok, but check if for example you are mapping the ID as self generate. I had the exact same problem, but I was sending an object with an ID different from 0.
Hope it helps!
My problem was this:
[Bind(Include="Name")] EventType eventType
Should have been:
[Bind(Include="EventTypeId,Name")] EventType eventType
Just as other answers suggest nhibernate was using zero as the id for my entity.
If you have a trigger on the table, it can be the reason. In this case, add inside it
SET ROWCOUNT 0;
SET NOCOUNT ON;
This error happened to me in the following way:
List < Device > allDevices = new List < Device > ();
//Add Devices to the list
allDevices.Add(aDevice);
//Add allDevices to database //Will work fine
// allDevices.Clear(); //Should be used here
//Later we add more devices
allDevices.Add(anotherDevice);
//Add allDevices to Database -> We get the error
//Solution to this
allDevices.Clear(); //Before adding new transaction with the oldData,
I am prototyping an idea on the iPhone but I am at the SQLite vs CoreData crossroads. The main reason is that I can't seem to figure out how to do grouping with core data.
Essentially I want to show the most recent item posted grouped by username. It is really easy to do in a SQL statement but I have not been able to make it work in core data. I figure since I am starting a new app, I might as well try to make core data work but this part is a major snag.
I added a predicate to my fetchrequest but that only gave me the single most recently added record and not the most recently added record per user.
The data model is pretty basic at this point. It uses the following fields:
username (string), post (string), created (datetime)
So long story short, are these types of queries possible with CoreData? I imagine that if SQLite is under the hood, there has to be some way to do it.
First of all, don't think of Core Data as another way of doing SQL. SQL is not "under the hood" of Core Data. Core Data deals with objects. Entity descriptions are not tables and entity instances are not records. Programming with Core Data has nothing to do with SQL, it merely uses SQL as one of several possible types of persistent stores. You don't deal with it directly and should never, ever think of Core Data in SQL terms.
That way lies madness.
You need drink a lot of tequila and punch yourself in the head repeatedly until you forget everything you ever knew about SQL. Otherwise, you will just end up with an object graph that is nothing but a big spread sheet.
There are several ways to accomplish what you want in Core Data. Usually you would construct fetch with a compound predicate that would return all post within a certain date range made by a specific user. Fetched results controllers are especially handy for this.
A most straightforward method would be to set up you object graph like:
UserEntity
--Attribute username
--Relationship post <-->> PostEntity
PostEntity
--Attribute creationDate
--Attribute content
-- Relationship user <<--> UserEntity
Then in your UserEntity class have a method like so:
- (NSArray *) mostRecentPost{
NSPredicate *recentPred=[NSPredicate predicateWithFormat:#"creationDate>%#", [NSDate dateWithTimeIntervalSinceNow:-(60*60*24)]];
NSSet *recentSet=[self.post filteredSetUsingPredicate:recentPred];
NSSortDescriptor *dateSort=[[NSSortDescriptor alloc] initWithKey:#"creationDate" ascending:NO];
NSArray *returnArray=[[recentSet allObjects] sortedArrayUsingDescriptors:[NSArray arrayWithObject:dateSort]];
return returnArray;
}
When you want a list of the most recent post of a particular user sorted by date just call:
NSArray *arrayForDisplay=[aUserEntityClassInstance mostRecentPost];
Edit:
...do I just pass each post block of
data (content,creationDate) to the
post entity? Do I also pass the
username to the post entity? How does
the user entity know when to create a
new user?
Let me pseudo code it. You have two classes that define instances of userObj and a postObj. When a new post comes in, you:
Parse inputPost for a user;
Search existing userObj for that name;
if userObj with name does not exist
create new userObj;
set userObj.userName to name;
else
return the existing userObj that matches the name;
Parse inputPost for creation date and content;
Search post of chosen userObj;
if an exiting post does not match content or creation date
create new postObj
set postObj.creationDate to creation date;
set postObj,content to content;
set postObj.user to userObj; // the reciprocal in userObj is created automatically
else // most likely ignore as duplicate
You have separate userObj and postObj because while each post is unique, each user may have many post.
The important concept to grasp is that your dealing with object i.e. encapsulated instance of data AND logic. This isn't just rows and columns in a db. For example, you could write managed object subclasses in which a single instance could decide whether to form a relationship with an instance of another class unless a specific internal state of the object was reached. Records in dbs don't have that sort of logic or autonomy.
The best way to get a handle on using objects graphs for data models is to ignore not only db but Core Data itself. Instead, set out to write a small test app in which you hand code all the data model classes. It doesn't have to be elaborate just a couple of attributes per class and a reference of some sort to the other class. Think about how you would manage parsing the data out to each class, linking the classes and their data together and then getting it back out. Do that by hand once or twice and the nature of object graphs becomes readily apparent.
There are other considerations that might tip your decision in the direction of SQLite versus Core Data with a SQLite store. I found myself nodding in agreement while reading a good blog post on the subject. I've found exactly the same thing (and am consequently moving a high-performance app away from Core Data): "Core Data is the right answer, except when it’s not..."
It's a great technology, but one size definitely does not fit all.
If 'posts' is a NSSet of User, you could get the last post with a predicate:
NSDate *lastDate = [userInstance valueForKeyPath:#"#max.date"];
NSSet *resultsTemp = [setOfPosts filteredSetUsingPredicate:[NSPredicate predicateWithFormat:#"fecha==%#", lastDate] ];
The resultsTemp set will contain an object of type Post which has the newest date.