I am using Listitemrenderer(Zk UI and Grails) to render a list of books from database table called "book". I have different types of books in the table. the book types are determined by the bookType attribute. The issue I am having is that when I do:
Book bookInstance = (Book) data
it shows null point exception when there are different types of books in the table. But in the case when there are books of a single type, it works fine.
Is there a way to filter the parameter 'data' of Listitemrenderer so that I could get books of a certain type I want? Again, there is an attribute bookType in database table that distinguishes different types of book.
Thank you!!!
you can use Query String with 'where' clause specifying 'book_type'.
this way use can fetch the desired result.
Related
How do I get records from a table with a specific ID and store these records in an Array?
For better understanding I try to explain it to you more clear:
Imagine a table with the following columns: (ID,FIX_ID,AMOUNT)
where ID is an unique ID which always will auto_incremented by 1.
FIX_ID is an ID which can appear multiple times in the table.
And AMOUNT is just a simple type which represents the amount of some "things".
So all I want to get now is every record from the table which have the FIX_ID that I am looking for.
Remember: The FIX_ID is not unique => it can appear multiple times.
And that's it. So imagine that I want to get all records with the FIX_ID of 10. All these records which I will get should be stored in an Array.
My question to you: Is it possible to realize this "request" to the database with ActiveRecord?
If so, then how?
You can get an ActiveRecord::Relation object like this:
ModelName.where('FIX_ID = ?', 10)
Relation object will perform a database query as soon as you call methods like all, each or any? on it. If you really need an array, call to_a on that Relation:
ModelName.where('FIX_ID = ?', 10).to_a
The scenario:
I have a considerable amount of entities as models in CodeFirst mapped to the database fieldname with the Attribute [Column("str")].
I have a bunch of Reporting Service Reports (in local-mode) with the DataSets mapped to the database field names.
I can't pass direct results of linq queries to those reports with the ToList() method because of the field names. What I can do (and I'm trying to avoid) is to type select new for each object; or run each query via a different datasource.
Question:
I would like to know if there is any trick to have a IQueryable object with the original field names instead of the property names. Something like a dynamic select new.
Any suggestions will be appreciated.
No, there isn't. The database column names either have to match the property name, or you have to use the Column attribute to make them line up. That's your only choices.
I'm beginning to use Core Data and I'm trying to get a solid understanding of how it would work in my case. I'm building an iPad photo album app that will allow users to filter their results based on keywords and a couple of other attributes associated with each photo. The attributes would be basic stuff like orientation and whether its color or not. Keywords will be based on a set of defined values that the user will have to choose from.
Here's a rough outline of what I have in the Core Data Model so far:
Entities (attributes):
Photos (id, title, desc, file path)
Keywords (id, name)
Since a photo can have multiple keywords associated with it, do I create a relationship from 'Photos' with a destination of 'Keywords' and make it a -to many relationship? Is that correct/crazy/completely wrong? How would I go about adding/fetching records for that relationship?
If the Keywords are defined and the user can choose them from a defined list, I wouldn't have used an entity Keyword. Tip always use singular names (Photo, Keyword). I would have the keywords in a plist and the user chooses a few, you could then create a comma separated string to save as a property on Photo. As Joseph mentioned, you want to subclass NSManagedObject and I would then add a method that receives a string (keyword) and returns a BOOL indicating if that keyword was part of the comma separated keyword values on the Photo property
This is correct. There are several approaches to accessing this, but the most straight-forward way is to generate NSManagedObject subclasses (Editor->Create NSManagedObject Subclass...). When populating these the Photo entity, the Keyword relationship will be populated in the property called keywords.
In Xcode you can add "Indexes" for an entity in the data model inspector.
For the screenshot I did hit "add" twice so "comma,separated,properties" is just the default value.
What exactly are those indexes?
Do they have anything to do with indexed attributes? And if they have what is the difference between specifying the Indexes in this inspector and selecting "Indexed" for the individual attribute?
Optimizing Core Data searches and sorts
As the title says, indexing is to speed up searching and sorting your database. However it slows down saving changes to persistant store. It matters when you are using NSPredicate and NSSortDescriptor objects within your query.
Let's say you have two entities: PBOUser and PBOLocation (many to many). You can see its properties at the image below:
Suppose that in database there is 10,000 users, and 50,000 locations. Now we need to find every user with email starting on a. If we provide such query without indexing, Core Data must check every record (basically 10,000).
But what if it is indexed (in other words sorted by email descending)? --> Then Core Data checks only those records started with a. If Core Data reaches b then it will stop searching because it is obvious that there are no more records whose email starts with a since it is indexed.
How to enable indexing on a Core Data model from within Xcode:
or:
Hopefully they are equivalent:-)
But what if you wanted: Emails started with a and name starts with b You can do this checking INDEXED for name property for PBOUser entity, or:
This is how you can optimise your database:-)
Use the Indexes list to add compound indexes to the entity. A compound index is an index that spans multiple attributes or relationships. A compound index can make searching faster. The names of attributes and relationships in your data model are the most common indexes. You must use the SQLite store to use compound indexes.
Adding a row with a single attribute to the Indexes list is equivalent to selecting Indexed for that attribute: It creates an index for the attribute to speed up searches in query statements.
The Indexes list is meant for compound indexes. Compound indexes are useful when you know that you will be searching for values of these attributes combined in the WHERE clause of a query:
SELECT * FROM customer WHERE surname = "Doe" AND firstname = "Joe";
This statement could make use of a compound index surname, firstname. That index would also be useful if you just search for surname, but not if you only search for firstname. Think of the index as if it were a phone book: It is sorted by surname first, then by first name. So the order of attributes is important.
How do I query the database to find objects that contain one or more attributes that are stored as serializable?
For example, I have a concert which occurs only in certain cities. I want to make a Concert object with a column called cities and store an array of cities.
If I want to query my database to find all concerts that occur in 1 city (or all concerts that occur in an array of n cities), how do I do this?
The best way to do this isn't to store it in a serialized column, but a separate table called Cities. Then you can do this:
City.find_by_name('Cityname').concerts
One possible way to query would be to use SQL's LIKE condition. This would work for boolean conditions in serialized tables.
For example to find those Users with the 'notification' option on,
users=User.arel_table
User.where(users[:options].matches("%notification: true%"))
As for other type of variables this would not be as feasible.