I am using Qt's Model View Programming on a database, where an object is represented using multiple tables. Assume the following object and coresponding database tables which perfectly fit my design:
TagObject
- id
- name
- usable
- information
tag_table
- id
- name
- usable
tag_info_table
- id_ref
- info
As you may see, the information property is separated into another table to prevent existence of NULL because this property is optional.
In the database I have a view which aggregates the values into one 'table' which can be queried using QSqlTableModel. Note that INSERTing data is not possible this way. As far as I could understand, the database design is not supported by Qt's classes, neither QSqlTableModel nor QSqlRelationalTableModel do support this. (Additionally QSqlQueryModel does not support inserts at all so this is out of question.)
Am I missing something? Is there any way to do this using Qt's SQL classes? Or is the only way to achive this subclassing QSqlQueryModel as pointed out here?
The model is read-only by default. To make it read-write, you must subclass it and reimplement setData() and flags(). Another option is to use QSqlTableModel, which provides a read-write model based on a single database table.
Edit: As for subclassing I found this reference as a nice entry point.
Ideally, the view should have appropriate triggers that will modify the underlying tables. Make the view writable and your problems go away: you can use a QSqlTableModel directly on that view, then.
Alternatively, you can have a QSqlTableModel for each table, and then write a custom proxy model that supports inserts and translates between the source models and forms the writable view. It'll be more work than writing the SQL triggers.
Related
I have a QSqlTableModel that works to view/update a table in mysql. If I make a view of the table then it will display the data but edits fail. I can update the table with a mysql update so the view is updateable.
I think what is happening is that since the view does not have a primary key the primary key for the QSqlTableModel is not set and it won't update.
I can't find any examples how to set the primary key. The setPrimaryKey() of QSqlTableModel is protected. I am sure I am missing something fundamental but searching returns a lot of results for view in the context of MVC.
Can anyone point me to an example of how to call setPrimaryKey or an example of using a QSqlTableModel to update a database view in mysql?
I am thinking now that this is not going to work like I want it to anyway. I could probably make it work for a view with a single table but it would likely be too much work to make it behave properly if I have a view with multiple tables. It would be better to take a different approach.
I have a table in another database I would like to scaffold via ASP.NET Dynamic Data and incorporate into my existing Entity Model - is there anyway to do this? (eg using a view or other mechanism or customize the view, edit or insert operations via ad-hoc SQL or stored procedures?)
I don't want to replicate the entire DynamicData sub-folder structure and create another entity model for just one table
I was able to solve this by manually creating an entity in the SSDL and CSDL sections of the .edmx file by using a DefiningQuery and then defining the EntitySets for my entity class
I also added insert / update / delete Function elements to the SSDL with inline SQL using the CommandText property
At this point I had enough to let the Designer map the CRUD methods to these inline SQL functions I defined
It's a little tricky but it works and the general approach opens up many possibilities I had not thought about
How do I define a database view using Entity Framework 4 Code-First? I can't find anything about this anywhere!
That's because you cannot define database view using code-first approach. Database view is database construct which uses SQL Query on top of existing tables / functions. You can't define such constructs using code first.
If you want view you must create it manually by executing CREATE VIEW SQL script for example in custom initializer - it will be similar like this answer. Just be aware that this will not help you if you want to map entity to a view. In such case you would probably have to first drop table created by EF and create view with the same name (I didn't try it but it could do the trick). Also be aware that not every view is udpatable so you will most probably get read only entity.
To do a view you create the model, then in the initializer you run the SQL statement to create the view directly against the context with the first line of code, and then in the context you override OnModelCreating and run the second line of code to ignore the model.
context.Database.ExecuteSqlCommand(Resources.<resourcename>);
modelBuilder.Ignore<modeltype>();
Often I need to combine data from multiple tables and display the result in a GridView control.
I can write a Linq query inline in the Page_load event, return an anonymous type that combines all the fields that I need, and databind the result to the GridView control.
Problem: I use 'helper methods' as described by Scott Guthrie on his blog. Such a helper method cannot return an anonymous type. The query would have to be inline for this approach.
I can write a database view that returns the data that I need, and write a helper method with a query against this (new and known) type that it returns.
Problem: I will need a lot of views in my database schema, and I will introduce a lot of redundant aspects of my data. I also lose some of the advantage of using Linq - removing all business logic from the database.
I would like to take an approach that lets me keep the Linq queries in helper methods, yet allows me to access all the attributes that I need on the grid in their respective databinding expressions. Can this be done?
I asked the wrong question, as I frequently do. What prompted me to look into anonymous types was an apparent limitation of the GridView - my inability to use a databinding expression in an <asp:BoundField> (the DataField parameter only accepts column names of the table that the Linq query pulls in).
Turns out that in a TemplateField it is possible to use Eval and access members of the Linq data item, and Linq takes care of the query for me.
In other words, I can keep the query in my helper method, have it return a primary database table type (e.g. Account), and I bind the Accounts to the GridView.
In the databinding expressions I can access data members of the Account objects that reside in other tables, without having to explicitly pull them in in the query. Perfect.
I don't know if there is a viable way to achieve this using anonymous types. But I have a suggestion that will work in WinForms, but I am not sure about ASP.NET.
What you need is a type with properties where neither the number of properties, nor the types and names of the properties are known at compile time. One way to create such a thing is ICustomTypeDescriptor.
You have to create a type implementing this interface with an private backing store of objects backing the properties returned by the query for one row from the query. Then you implement GetProperties() to return one PropertyDescriptor per column and PropertyDescriptor.GetValue() and PropertyDescriptor.SetValue() to access the backing array.
By implementing PropertyDescriptor.Name you will get the correct column name; this will probably require another backing store storing the property names. And there is a lot more to implement, but in the end your new type will behave almost like a normal type - and now the if - if the control you are binding to knows about and uses ICustomTypeDescriptor.
UPDATE
I just found an bit of text stating that ASP.NET data binding knows and uses ICustomTypeDescriptor.
Scott's earlier post in the series talks about shaping the result set before inserting into a grid:
Part 3 - Querying our Database
Scroll down to "Shaping our Query Results".
Is it possible to add properties to my model that dont exist in the database?
For example I have a calendar table, I want to retireve this data in my MVC controller then work out time left until each entry starts and return this to the view. So I would like another property in my calendar model to hold time left which is a value that I will generate outside of the database.
I've tried just adding the property but when I do that I get errors because the property is not mapped to anything.
Is this even possible?
Thanks
You should be able to add the property to the Model but you will not be able to query it with LINQ. LINQ will ultimately build and expression which it will want to run against the database using SQL. Its at that point that your LINQ will fail to find a mapping from your property to a field somewhere.
If your query returns an IEnumerable of the actual type on which you have created the property your view may be able to access it. I can't remember if EF insists on mapping in that case, it may do.
You might find that you can create subsequent LINQ query that uses LINQ-to-objects if you want to provide some other composite type to your view.
It's a non-persistent property or transient. I don't know Entity Framwork well but with a quick google search you should find the answer.
BTW you can find a lot of tips here :
http://weblogs.asp.net/zeeshanhirani/archive/2008/12/18/my-christmas-present-to-the-entity-framework-community.aspx
After making a quick search myself and a test in VS2008 I don't see a way to exclude a property from the mapping. Maybe it requires you to edit manually the mapping file ? :(