I'm new to using ObjectBox, so I've been trying to do some experimenting with its query system to familarize myself with it. One of the queries I've been unable to do is a query comparing two properties. Ignoring the errors they throw, these are some examples of what I'm looking to do:
// Get objects where first number is bigger than second number
boxA.query(ObjectA_.firstNumber.greaterThan(ObjectA_.secondNumber))
// Get parent objects where one of its children has a specific value from the parent
parentBox.query().linkMany(ParentObject_.children, ChildObject_.name.equals(ParentObject_.favoriteChild));
I know based on this question that it's possible in Java using filters, but I also know that query filters are not in ObjectBox for Dart. One of the workaround I've been testing is querying for one property, getting the values, and using each value to query for the second property. But that becomes unsustainable at even moderately sized amounts of data.
If anyone knows of a "proper" way to do this without the use of Java filters, that would be appreciated. Otherwise, if there's a more performant workaround than the one I came up with, that would be great too.
There is no query filter API for Dart in ObjectBox, because Dart already has the where API.
E.g. for a result list write results.where((a) => a.firstNumber >= a.secondNumber).
Related
Beginner question. I would like to have a value list display only the records in a found set.
For example, in a law firm database that has two tables, Clients and Cases, I can easily create value list that displays all cases for clients.
But that is a lot of cases to pick from, and invites user mistakes. I would like the selection from the value list to be restricted to cases matched to a particular client.
I have tried this method https://support.claris.com/s/article/Creating-conditional-Value-Lists-1503692929150?language=en_US and it works up to a point, but it requires too much entry of data and too many tables.
It seem like there ought to be a simpler method using the find function. Any help or ideas greatly appreciated.
Currently using EF Core 3.1 (although upgrading to EF Core 5 soon, so if this becomes easier in that version, I'm still interested).
I'm implementing some global query filters to restrict access to entity data based on various criteria.
One of the filters operates on a type called "Encounter". An Encounter is linked to a single Person (via foreign key PersonID), but a Person can have many Encounters. The Person record has a navigation property to all linked encounters via Person.Encounters.
The query filter for Encounter is to restrict records based on the value of a particular property on Encounter (call it EncounterType). This works fine if my query filter is something like this:
x => x.EncounterType == "MyType"
However, I need to extend this logic so that an Encounter is allowed/loaded if any encounter linked to the Person meets the criteria.
So, the query filter for Encounter needs to be something like this:
x => x.Person.Encounters.Any(y => y.EncounterType == "MyType")
This does not currently work, because we run into a cycle and a StackOverflowException1; the global query filter for Encounter ends up expanding itself infinitely. (I think it's because we access the Person.Encounters navigation property and evaluate the same Encounter query filter for each encounter in Person.Encounters.)
What I really want to do is to completely disable the global query filter for any navigation properties used in this particular expression. In this scenario, I want to consider all other Encounters linked to the Person, without further filtering.
I know when actually running a query, I can call IgnoreQueryFilters(). I want something like that, but available inside the expression or when adding the query filter with HasQueryFilter().
Is this possible? Is there any other way I can accomplish this with global query filters?
[1] Which, while frustrating, is pretty cool for me...I've never posted a question about an actual stack overflow on StackOverflow :)
I have a use case where I need to get list of Objects from mongo based off a query. But, to improve performance I am adding Pagination.
So, for first call I get list of say 10 Objects, in next I need 10 more. But I cannot use offset and pageSize directly because the first 10 objects displayed on the page may have been modified [ deleted ].
Solution is to find Object Id of last object passed and retrieve next 10 objects after that ObjectId.
Please help how to efficiently do it using Morphia mongo.
Using morphia you can do this by the following command.
datastore.find(YourClass.class).field(id).smallerThan(lastId).limit(10).order("-ts");
Since you are querying for retrieving the items after the last retrieved id, you won't be bothered to deal with deleted items.
One thing I have thought up of is that you will have the same problem as with using skip() here unless you intend to change how your interface works.
Using ranged queries like this demands that you use a different kind of interface since it is must harder to detect now exactly what page you are on and how many pages exist in the future, especially if you are doing this to avoid problems with conventional paging.
The default type of interface to arise from this type of paging is merely a infinitely scrolling page, think of YouTube video comments or Facebook wall feed or even Google+. There is no physical pagination or "pages", instead you have a get more button.
This is the type of interface you will need to use to get ranged paging working better.
As for the query #cubbuk gives a good example:
datastore.find(YourClass.class).field(id).smallerThan(lastId).limit(10).order("-ts");
Except it should be greaterThan(lastId) since you want to find everything above that last _id. I would also sort by _id unless you make your OjbectIds sometime before you insert a record, if this is the case then you can use a specific timestamp set on insert instead.
This question is about why I would use the above keywords. I've found plenty of MSDN pages that explain how. I'm looking for the why.
What query would I be trying to write that means I need them? I ask because the examples I have found appear to be achievable in other ways...
To try and figure it out myself, I created a very simple entity model using the Employee and EmployeePayHistory tables from the AdventureWorks database.
One example I saw online demonstrated something similar to the following Entity SQL:
SELECT VALUE
DEREF(CREATEREF(AdventureWorksEntities3.Employee, row(h.EmployeeID))).HireDate
FROM
AdventureWorksEntities3.EmployeePayHistory as h
This seems to pull back the HireDate without having to specify a join?
Why is this better than the SQL below (that appears to do exactly the same thing)?
SELECT VALUE
h.Employee.HireDate
FROM
AdventureWorksEntities3.EmployeePayHistory as h
Looking at the above two statements, I can't work out what extra the CREATEREF, DEREF bit is adding since I appear to be able to get at what I want without them.
I'm assuming I have just not found the scenarios that demostrate the purpose. I'm assuming there are scenarios where using these keywords is either simpler or is the only way to accomplish the required result.
What I can't find is the scenarios....
Can anyone fill in the gap? I don't need entire sets of SQL. I just need a starting point to play with i.e. a brief description of a scenario or two... I can expand on that myself.
Look at this post
One of the benefits of references is that it can be thought as a ‘lightweight’ entity in which we don’t need to spend resources in creating and maintaining the full entity state/values until it is really necessary. Once you have a ref to an entity, you can dereference it by using DEREF expression or by just invoking a property of the entity
TL;DR - REF/DEREF are similar to C++ pointers. It they are references to persisted entities (not entities which have not be saved to a data source).
Why would you use such a thing?: A reference to an entity uses less memory than having the DEFEF'ed (or expanded; or filled; or instantiated) entity. This may come in handy if you have a bunch of records that have image information and image data (4GB Files stored in the database). If you didn't use a REF, and you pulled back 10 of these entities just to get the image meta-data, then you'd quickly fill up your memory.
I know, I know. It'd be easier just to pull back the metadata in your query, but then you lose the point of what REF is good for :-D
I have a core data application and I would like to get results from the db, based on certain parameters. For example if I want to grab only the events that occured in the last week, and the events that occured in the last month. Is it better to do a fetch for the whole entity and then work with that result array, to create arrays out of that for each situation, or is it better to use predicates and make multiple fetches?
The answer depends on a lot of factors. I'd recommend perusing the documentation's description of the various store types. If you use the SQLite store type, for example, it's far more efficient to make proper use of date range predicates and fetch only those in the given range.
Conversely, say you use a non-standard attribute like searching for a substring in an encrypted string - you'll have to pull everything in, decrypt the strings, do your search, and note the matches.
On the far end of the spectrum, you have the binary store type, which means the whole thing will always be pulled into memory regardless of what kind of fetches you might do.
You'll need to describe your managed object model and the types of fetches you plan to do in order to get a more specific answer.