I've got 2 classes
Reports - objectID, Title, Date & relationItem ( Relation type column linked up to Items)
Items - ObjectID, Title, Date etc
I want to query all the Items that are equal to a objectID in reports. Users create reports then add items to them. These items are found in the Items table.
I've looked at the https://parseplatform.github.io/docs/ios/guide/#relations but don't see anything for swift3.
I've tried a few things with little success. This snipplet below i did find, but not sure how to apply it to my classes.
var relation = currentUser.relationForKey("product")
relation.query()?.findObjectsInBackgroundWithBlock({
Would love somebody to direct me into the right direction! Thanks
Tried this code below too!
var query = PFQuery(className:"Items")
query.whereKey("relationItem ", equalTo: PFObject(withoutDataWithClassName:"Reports", objectId:"MZmMHtobwQ"))
Ok so i had to change the table slightly to get this to work to prevent a query within a query.
I've added a relation Type to the Items table instead of the Reports Table
Then i managed to retrieve all the Items based of that report ObjectId like this:
let query = PFQuery(className:"Items")
query.whereKey("reportRelation", equalTo: PFObject(withoutDataWithClassName:"Reports", objectId:"3lWMYwWNEj"))
This then worked. Note that reportRelation is the Relational Type Column.
Thanks
When you’re thinking about one-to-many relationships and whether to implement Pointers or Arrays, there are several factors to consider. First, how many objects are involved in this relationship? If the “many” side of the relationship could contain a very large number (greater than 100 or so) of objects, then you have to use Pointers. If the number of objects is small (fewer than 100 or so), then Arrays may be more convenient, especially if you typically need to get all of the related objects (the “many” in the “one-to-many relationship”) at the same time as the parent object.
http://parseplatform.github.io/docs/ios/guide/#relations
If you are working with one to many relation, use pointer or array. See the guide for examples and more explanation.
Related
Suppose I have a large (300-500k) collection of text documents stored in the relational database. Each document can belong to one or more (up to six) categories. I need users to be able to randomly select documents in a specific category so that a single entity is never repeated, much like how StumbleUpon works.
I don't really see a way I could implement this using slow NOT IN queries with large amount of users and documents, so I figured I might need to implement some custom data structure for this purpose. Perhaps there is already a paper describing some algorithm that might be adapted to my needs?
Currently I'm considering the following approach:
Read all the entries from the database
Create a linked list based index for each category from the IDs of documents belonging to the this category. Shuffle it
Create a Bloom Filter containing all of the entries viewed by a particular user
Traverse the index using the iterator, randomly select items using Bloom Filter to pick not viewed items.
If you track via a table what entries that the user has seen... try this. And I'm going to use mysql because that's the quickest example I can think of but the gist should be clear.
On a link being 'used'...
insert into viewed (userid, url_id) values ("jj", 123)
On looking for a link...
select p.url_id
from pages p left join viewed v on v.url_id = p.url_id
where v.url_id is null
order by rand()
limit 1
This causes the database to go ahead and do a 1 for 1 join, and your limiting your query to return only one entry that the user has not seen yet.
Just a suggestion.
Edit: It is possible to make this one operation but there's no guarantee that the url will be passed successfully to the user.
It depend on how users get it's random entries.
Option 1:
A user is paging some entities and stop after couple of them. for example the user see the current random entity and then moving to the next one, read it and continue it couple of times and that's it.
in the next time this user (or another) get an entity from this category the entities that already viewed is clear and you can return an already viewed entity.
in that option I would recommend save a (hash) set of already viewed entities id and every time user ask for a random entity- randomally choose it from the DB and check if not already in the set.
because the set is so small and your data is so big, the chance that you get an already viewed id is so small, that it will take O(1) most of the time.
Option 2:
A user is paging in the entities and the viewed entities are saving between all users and every time user visit your page.
in that case you probably use all the entities in each category and saving all the viewed entites + check whether a entity is viewed will take some time.
In that option I would get all the ids for this topic- shuffle them and store it in a linked list. when you want to get a random not viewed entity- just get the head of the list and delete it (O(1)).
I assume that for any given <user, category> pair, the number of documents viewed is pretty small relative to the total number of documents available in that category.
So can you just store indexed triples <user, category, document> indicating which documents have been viewed, and then just take an optimistic approach with respect to randomly selected documents? In the vast majority of cases, the randomly selected document will be unread by the user. And you can check quickly because the triples are indexed.
I would opt for a pseudorandom approach:
1.) Determine number of elements in category to be viewed (SELECT COUNT(*) WHERE ...)
2.) Pick a random number in range 1 ... count.
3.) Select a single document (SELECT * FROM ... WHERE [same as when counting] ORDER BY [generate stable order]. Depending on the SQL dialect in use, there are different clauses that can be used to retrieve only the part of the result set you want (MySQL LIMIT clause, SQLServer TOP clause etc.)
If the number of documents is large the chance serving the same user the same document twice is neglibly small. Using the scheme described above you don't have to store any state information at all.
You may want to consider a nosql solution like Apache Cassandra. These seem to be ideally suited to your needs. There are many ways to design the algorithm you need in an environment where you can easily add new columns to a table (column family) on the fly, with excellent support for a very sparsely populated table.
edit: one of many possible solutions below:
create a CF(column family ie table) for each category (creating these on-the-fly is quite easy).
Add a row to each category CF for each document belonging to the category.
Whenever a user hits a document, you add a column with named and set it to true to the row. Obviously this table will be huge with millions of columns and probably quite sparsely populated, but no problem, reading this is still constant time.
Now finding a new document for a user in a category is simply a matter of selecting any result from select * where == null.
You should get constant time writes and reads, amazing scalability, etc if you can accept Cassandra's "eventually consistent" model (ie, it is not mission critical that a user never get a duplicate document)
I've solved similar in the past by indexing the relational database into a document oriented form using Apache Lucene. This was before the recent rise of NoSQL servers and is basically the same thing, but it's still a valid alternative approach.
You would create a Lucene Document for each of your texts with a textId (relational database id) field and multi valued categoryId and userId fields. Populate the categoryId field appropriately. When a user reads a text, add their id to the userId field. A simple query will return the set of documents with a given categoryId and without a given userId - pick one randomly and display it.
Store a users past X selections in a cookie or something.
Return the last selections to the server with the users new criteria
Randomly choose one of the texts satisfying the criteria until it is not a member of the last X selections of the user.
Return this choice of text and update the list of last X selections.
I would experiment to find the best value of X but I have in mind something like an X of say 16?
I'm a Seam newbie in an already established project, so a lot of code I use is borrowed and I'm not always fully sure how things work. My problem is that I am using a query object extended from EntityQuery to back a list page with search and sort capabilities that needs to search across a many-to-many relationship and a separate many-to-one relationship which must also be used to sort. Because the many-to-many relationship has to be joined in to allow for the search capability, the query returns duplicate records for each assignment. That's not a big deal because I just added "distinct" to the ejbql and that worked fine. However, when I try to order by the other many-to-one relationship, Oracle throws an error. It appears that Oracle will not accept an order by column that is not in the select clause when using the distinct keyword http://ora-01791.ora-code.com/, and http://oraclequirks.blogspot.com/2009/04/ora-01791-not-selected-expression.html.
Here are the relationships as they are defined in the entities: [Subject m:m JobFunction] (obviously through an assignment table [Subject o:m Subject_JobFunction m:o JobFunction]), and [Subject m:o Type]. Because I need to search Subject by JobFunction, it is joined in in the ejbql which requires the distinct keyword to only return distinct Subjects to the list page. When I try to order by the Type.name (through the many-to-one relationship), the resulting query makes Oracle angry and throws the "ORA-01791: not a SELECTed expression" error. SubjectQuery code:
#Override
public String getEjbql() {
return "select subject from Subject subject left outer join subject.jobFunctions as jobFunction";
}
#Override
#SuppressWarnings("rawtypes")
public List<ValueExpression> getRestrictions() {
ValueExpression[] RESTRICTIONS = {
createValueExpression("lower(subject.name) like #{subjectQuery.prepRestriction(subjectQuery.subject.name)}"),
createValueExpression("subject.active = #{subjectQuery.active}"),
createValueExpression("subject.type.name = #{subjectQuery.typeName}"),
createValueExpression("jobFunction.name = #{subjectQuery.jobFunctionName}")
};
return Arrays.asList(RESTRICTIONS);
}
When I set the query order when a user sorts by the Type name through the front end:
"#{subjectQuery.order=='UPPER(subject.type.name) asc'}"
I get the Oracle error. If I take the distinct out of the ejbql, the sort works fine, but I get duplicate Subject records. When I add the distinct keyword the list works fine without duplicate records, but the sort throws an error. Does anyone have any suggestions about how I can restructure the ejbql to return distinct records without the distinct keyword to make the sort happy, or how to do the sort without making Oracle angry that the sort column referenced in the query is not in the select clause? I have read several places that my answer might be in the the Hibernate Criteria API, but I have no idea how to leverage it in the context of an extended EntityQuery class with what I am trying to accomplish. Please Help!
If you are adding a DISTINCT, then something is broken.
"Because the many-to-many relationship has to be joined in to allow for the search capability, the query returns duplicate records for each assignment. "
Consider the case that a person can work on many projects and a project can have many persons. There is a uniqueness of a 'person/project'. If you want a list of people that work in either project A or B (or both) then you may get
FRED/PROJ_A
BILL/PROJ_A
FRED/PROJ_B
TOM/PROJ_B
BILL/PROJ_C
If you only show the names (not the projects), you can still order by project, but you will see
FRED
BILL
FRED
TOM
BILL
If you do a DISTINCT, you can no longer order by project, because you don't know whether the FRED is the one from PROJ_A or PROJ_B or whether BILL comes before TOM (based on PROJ_A) or after TOM (based on PROJ_C).
So remove the DISTINCT and always show the column on which you are ordering (because then you'll see why the duplicates aren't actually duplicates).
I'm not sure how the generated query(ies) is(are) different, but I found an answer. I wasn't aware of the fetch command for hibernate, which fixes the need for the distinct keyword (again, not sure exactly how, maybe by subquerying?). After changing the ejbql to:
#Override
public String getEjbql() {
return "select subject from Subject subject left join fetch subject.jobFunctions jobFunction";
}
the distinct is no longer needed and therefore, Oracle does not complain about the order by column not being in the select clause. The list works as expected and the sort column works! Yay!
Predictably, I found the answer here on stackoverflow. The question was not exactly the same, but the hql syntax worked for me: HQL order by within a collection
I am building a school management app where they track student tardiness and absences. I've got three entities to help me in this. A Students entity (first name, last name, ID, etc.); a SystemAbsenceTypes entity with SystemAbsenceTypeID values for Late, Absent-with-Reason, Absent-without-Reason; and a cross-reference table called StudentAbsences (matching the student IDs with the absence-type ID, plus a date, and a Notes field).
What I want to do is query my entities for a given student, and then add up the number of each kind of Absence, for a given date range. I prepare my currentStudent object without a problem, then I do this...
Me.Data.LoadProperty(currentStudent, "StudentAbsences") 'Loads the cross-ref data
lblDaysLate.Text = (From ab In currentStudent.StudentAbsences Where ab.SystemAbsenceTypes.SystemAbsenceTypeID = Common.enuStudentAbsenceTypes.Late).Count.ToString
...and this second line fails, complaining "Object reference not set to an instance of an object."
I presume the problem is that while it DOES see that there are (let's say) four absences for the currentStudent (ie, currentStudent.StudentAbsences.Count = 4) -- it can't yet "peer into" each one of the absences to look at its type. In fact, each of the four StudentAbsence objects has a property called SystemAbsenceType, which then finally has the SystemAbsenceTypeID.
How do I use .Expand or .LoadProperty to make this happen? Do I need to blindly loop through all these collections, firing off .LoadProperty on everything before I can do my query?
Is there some other technique?
When you load the student, try expanding the related properties.
var currentStudent = context.Students.Expand("StudentAbsences")
.Expand("StudentAbsences/SystemAbsenceTypes")
.Where(....).First();
Background
I'm writing an adapter for ESE to .NET and LINQ in a Google Code project called eselinq. One important function I can't seem to figure out is how to get a list of indexes defined for a table. I need to be able to list available indexes so the LINQ part can automatically determine when indexes can be used. This will allow much more efficient plans for user queries if appropriate indexes can be found.
There are two related functions for querying index information:
JetGetTableIndexInfo - get index information by tableID
JetGetIndexInfo - get index information by tableName
These only differ in how the related table is specified (name or tableid). It sounds like these would support the function I want but all the info levels seem to require that I already have a certain index to query information for. The only exception is JET_IdxInfoCount, but that only counts how many indexes are present.
JET_IdxInfo with its JET_INDEXLIST sounds plausible but it only lists the columns on a specific index.
Alternatives
I am aware that I could get the index information another way, like annotations on .NET types corresponding to database tables, or by requiring a index mapping be provided ahead of time. I think there's enough introspection implemented to make everything else work out of the box without the user supplying extra information, except for this one function.
Another option may be to examine the system tables to find related index objects, but this is would mean depending on an undocumented interface.
To satisfy this question, I want a supported method of enumerating the indexes (just the name would be sufficient) on a table.
You are correct about JetGetTableIndexInfo and JetGetIndexInfo and JET_IdxInfo. The twist is that the data is returned in a somewhat complex: a temporary table is returned containing a row for the index and then a row for each column in the table. To just get the index names you will need to skip the column rows (the column count is given by the value of the columnidcColumn column in the first row).
For a .NET example of how to decipher this, look at the ManagedEsent project. In the MetaDataHelpers.cs file there is a method called GetIndexInfoFromIndexlist that extracts all the data from the temporary table.
So I'm extremely new to Linq in .Net 3.5 and have a question. I use to use a custom class that would handle the following results from a store procedure:
Set 1: ID Name Age
Set 2: ID Address City
Set 3: ID Product Price
With my custom class, I would have received back from the database a single DataSet with 3 DataTables inside of it with columns based on what was returned from the DB.
My question is how to I achive this with LINQ? I'm going to need to hit the database 1 time and return multiple sets with different types of data in it.
Also, how would I use LINQ to return a dynamic amount of sets depending on the parameters (could get 1 set back, could get N amount back)?
I've looked at this article, but didn't find anything explaining multiple sets (just a single set that could be dynamic or a single scalar value and a single set).
Any articles/comments will help.
Thanks
I believe this is what you're looking for
Linq to SQL Stored Procedures with Multiple Results - IMultipleResults
I'm not very familiar with LINQ myself but here is MSDN's site on LINQ Samples that might be able to help you out.
EDIT: I apologize, I somehow missed the title where you mentioned you wanted help using LINQ with Stored Procedures, my below answer does not address that at all and unfortunately I haven't had the need to use sprocs with LINQ so I'm unsure if my below answer will help.
LINQ to SQL is able hydrate multiple sets of data into a object graph while hitting the database once. However, I don't think LINQ is going to achieve what you ultimately want -- which as far as I can tell is a completely dynamic set of data that is defined outside of the query itself. Perhaps I am misunderstanding the question, maybe it would help if you provide some sample code that your existing application is using?
Here is a quick example of how I could hydrate a anonymous type with a single database call, maybe it will help:
var query = from p in db.Products
select new
{
Product = p,
NumberOfOrders = p.Orders.Count(),
LastOrderDate = p.Orders.OrderByDescending().Take(1).Select(o => o.OrderDate),
Orders = p.Orders
};