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();
Related
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.
Consider two types of documents Company and Person:
Company has 2 fields:
name of type String
employees of type List of Person
Person has 2 fields:
name of type String
city of type String
How can I create a query where I find all the companies who have at least N employees in a given city ?
EDIT: In other words, How is it possible to do something like this with CouchBase Lite.
I think there are several ways to approach this.
One suggestion is to create a view that, when given a Company document, emits a key/value pair via the map stage. The key could be a map containing the company name and city, and the value could be anything (e.g. employee name). Then add a reduce function that sums all the index entries (that's what the first part creates) with the same key.
So the result of the reduce stage output for the view is the total number of employees keyed by company + city. You can then do queries to get your result.
Views and Queries are really powerful, but can take some thought. Focus on getting the information you need out of the View, so you can query flexibly.
Take a look at the View and Query documentation for more details.
I'm working on a VB6 program that connects to a SQL Server 2008 R2 database. In the past I have always used the MSFlexGrid control and populated it manually. Now, however, the guy who is paying me for this wants me to use data-bound grids instead, which forces me to use the MSHFlexGrid control because I'm using ADO and not DAO. So, I have two questions...
First, how would I move a column in a MSHFlexGrid? For example, if I wanted the third column to appear as the sixth column in the grid, is there a simple single line of code that would do that?
Second, believe it or not, I've never had to do anything in a grid other than display the data, as is, from a recordset. Now, however, I have a recordset with some fields that contain just ID numbers that refer to records in other files - for example, a field containing an ID number referring to a record in the Customers table, instead of the field containing the customer's name. What is the easiest way to, instead of having a column showing customer ID numbers from the recordset, having that column show customer names? I thought I read somewhere that there's a way to embed a sql command in a MSHFlexGrid column, but if there is I wouldn't know how to do it. Is this possible, or is there a simpler way to do it?
TIA,
Kevin
The column order would typically be handled by your SELECT statement.
Say you have a Pies table that has a FruitID foreign key related to the FruitID in a Fruits table:
SELECT PieID AS ID, Pie, Fruit FROM Pies LEFT OUTER JOIN Fruits
ON Pies.FruitID = Fruits.FruitID
This returns 3 items: ID, Pie, and Fruit in that order.
Moving columns after the query/display operation is rarely used, but yes ColPosition can be used for that.
Wow! VB6.... Back to the future! :-)
You can move Columns using the ColPosition Property.
This article shows how you could setup the grid to display hierarchical data.
If you just want to display the customer name on the same line as the main data then that is doable as well by just creating the proper SQL for your data source. For that matter you can control the column order the same way as well.
Now, how about considering upgrading to .Net? Just kidding..... No, I'm not. OK. I am, maybe. :-)
I have an Infragistics Ultragrid that is being used to display a list of attributes. Sometimes the attribute is an array so I am adding a sub row for each element so the user can optionally expand the row showing the array attribute and see all the element values.
So for each element I use:
var addedRow = mGrid.DisplayLayout.Bands[1].AddNew();
which if I have 300 elements gets called 300 times and takes around 9 seconds (I have profiled the application and this call is taking 98% of the elapsed time)
Is there a way to add these sub rows more efficiently?
I know I'm late with an answer, but hopefully someone can use my answer anyway. Whenever I need to set rows and subrows for ultragrid, I simply set the datasource by using linq and anonymous types to generate the propper collection.
say you have a list of persons (id, Name), and a list of cars (id, CarName, and OwnerId (personId))
now you like to show a gridview showing all persons, with an expandabel subrow providing which cars they own. simply do the following.
List<Person> persons = GetAllPersons();
List<Car> cars = GetAllCars();
grid.DataSource = persons.Select(x => new {x.Id, x.Name, Cars = cars.Where(z => z.OwnerId == x.Id).ToList()}).ToList();
Note the anonymous type I make, this will generate a list of objects having an id, Name, and a collection of cars. Also note that I call the ToList method twice in the last line, this is necessary in order to get ultragrid to bind properly.
Note further more that if you need to edit the gridview, the above method migth not be sufficient, as the ultragrid needs an underlaying datasource for modifying, and I dont believe that this will cope. BUT on the internet you'll find some extensions that can copy a Linq collection into a DataTable, doing that and then you should also be able of editing the grid.
I have often used the above method and it performs extremely well, even for huge collections.
Hope this helps somebody
you might want to use ultraGrid1.BeginUpdate(); and ultraGrid1.EndUpdate(true); to stop screen from repainting. made huge performance benefit for my app.
Also in my case I was populating nearly >10,000 rows, so have used UltraDataSource
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
};