I'm new to Swift/CoreData and SQl databases. I have a CoreDatabase with over 7000 items. I want to create an entity (or any other way) to store how often a certain entry in the DB has been used. I need this in order to create a weighted sorting algorithm that suggests certain entries.
The catch is that I do not want to store this on the entries themselves, they need to remain generic in order for me to be able to update them every now and again via my own Node server. So all users have the same DB. Whenever the user picks one of the items it's counter increments by one. Whenever I query an item the frequency should come with it so I can perform a sorting algorithm on it.
I've been reading up on articles, it seems like this can be done, but none so far have been really useful. I've also looked a SQLite articles on this but haven't found what I was looking for.
I'm thinking something along these lines:
FrequencyList { Item_1 { ...7000 items....
item_1_freq : 0, ------------> frequency : 0,
item_2_freq : 12, name: "lala"
item_3_freq : 3 ...
... };
...
7000?!?!
};
Or would a separate 'meta' entity in a one-to-one relationship with it's respective Item be a good solution?
How can I tackle this?
In Core Data it would probably be better like this:
Put the selection in an entity with a count property, and have a relationship between the selection and the item. The Selection --> Item relationship could be to-one or to-many depending on your needs; I have it as to-many here but that might not be best in your case.
If you want to get the number of selections for an Item, use the value of selection.count. Update selection.count when a selection occurs.
Related
I have a document based core data app with entity "Languages". This entity has two permanent attributes "key" and "comments".
Is it possible programmatically add and remove additional attributes during runtime ("language_1", "language_2", etc.) ?
My goal is to avoid creating table with let say 50 attributes when user needs only few (I don't know upfront how many attributes will be necessary).
Or maybe I should choose other solution ? :)
EDIT
Case explanation:
When user creates new document, table "Languages" has only 2 attributes "key" and "comments". During working with the document user can any time add or remove language(s) - I mean attributes (columns) not rows in the table.
My goal is to have dynamic entity like below.
Yes, it's possible. But it's probably not what you want. You'd have to recreate the amended Managed Object Model, for each document, at runtime whenever the document is opened.
After seeing your sketch, I suggest a slightly different model. By the way, best style is to use singular nouns for Entities ("Section", not "Sections), plural nouns for to-many Relationships ("sections", not "relSection"), and omit the entity name in its attributes ("comment", not "sectionComment").
Use one Entity for your permanent attributes. Call it "Word". Word has attributes "comments" and "key", and to-many relationships "translations" and "sections". On the other end of the "translations" relationship is a Translation entity, which has attributes "text" and also perhaps the name of the language (either as a string or as another relationship).
Something like this:
For your first example, you'd have one instance of Word, 3 instances of Translation (.text = Home, Zuhause, and Casa), and 3 instances of Language (.name = English, German, Spanish). When you add the second line, you'll get 1 more instance of Word, 3 more instances of Translation, but 0 more Languages. Add the new Translation instances to the existing Language's "translations" relationship instead.
Problem: When adding an object "Order" to my dbcontext, all nested objects of the order gets "readded" to the database, though the nested objects is static data and only a reference shoudl be added in the database.
Example:
The database holds 0 orders, and 3 items.
I add one order with 2 items.
Now the database hold 1 order, and 5 items. The two items in the order has been "readded" to the database, even though the items had the right primary keys before db.SaveChanges().
I realize that i may be able to attach the existing items to the dbcontext before saving changes, but is that really the only way to go? Can't EF figure out that to item already exists when the primary key matches an existing item?
Does anyone know if this is different in the new version of EF CodeFirst?
No EF cannot figure if entities are existing one or new one - both Add and Attach commands are graph oriented operations. You call them on one entity in the graph and they traverse all relations (and their relations and so on) and perform the operation for them as well.
You must figure correct state of each entity in the graph for example by using:
dbContext.Orders.Add(newOrder);
foreach(var item in newOrder.Items) {
dbContext.Entry(item).State = EntityState.Unchanged;
}
dbContext.SaveChanges();
You can use the reverse operation by calling Attach(newOrder) and set the order to Added state. The main difference will come with independent associations (for example many-to-many relations). The first approach will correctly add new relation between order and each item whereas second will not unless you manually set each relation to Added state (and changing state for relations is more complex).
Background is the team i'm in has just started using the EntityFramework; first we designed the database, put all the table relationships in place, foreign keys, etc; then thru visual studio add a new ADO.NET Entity Data Model, and auto-magically we get the generated edmx file representing the whole database !
Now i focus on two tables that provide data for all dropdowns and lookup lists;
TLookupDomain (domainID, domainName, domainDesc )
TLookup (lookupID, domainID, lookupCode, lookupDisplay, lookupDesc, sortOrder)
Relationship is 1-M going from left to right:
TLookupDomain -< TLookup -< TOther (+ another 30 or so other tables)
So lookupID is a foreign-Key to as many as 30 tables;
IQueryable<TLookup> qList = from l in ctx.TLookups
where l.domainID == 24
select l;
foreach (TLookup l in qList)
{
//do something.
System.Diagnostics.Debug.WriteLine("{0}\t{1}", l.lookupCode, l.lookupDisplay);
foreach (TOther f in l.TOthers)
{
System.Diagnostics.Debug.WriteLine("{0}\t{1}", f.feeAmount, f.feeDesc);
}
}
When i execute the above LINQ, i get all the fields for TLookup table (which is fair), BUT data is also fetched for the 30 or so tables that are linked to it, even though i am NOT interested in the other table's data at this point, and i am going to discard all data soon as LINQ fetches it.
Two Questions i have:
Q.1) Can i somehow modify the LINQ query above or tell the EntityFramework otherwise not to bother fetchin data from the 30 other linked tables ?
Q.2) is it "right" to have one edmx file that models the entire database? (sounds dodgy to me).
Configure Lazy Load to true for the model. Relations should be loaded only upon navegation. You can also split the models to avoid too many unnecessary relations.
Linq-to-Entities queries do not fetch anything automatically. Fetching of navigation properties is performet either by eager or lazy loading. You are not using eager loading because that requires calling Include in query (or ctx.LoadProperty separately). So if your data are fetched it must be due to lazy loading wich is enabled by default. Lazy loading triggers once you access the navigation property in the code.
You can also return only the data you need by using projections. Something like this should return readonly data:
var query = from l in ctx.TLookups
where l.domainId == 24
select new
{
l.lookupCode,
l.lookupDisplay,
l.TOthers
};
Having one or more EDMX is common dilemma. Working with single EDMX makes things more simple. If you want to know how to use multiple EDMXs and share conceptual definitions check these two articles: Part 1, Part 2.
Please help an EF n00b design his database.
I have several companies that produce several products, so there's a many-to-many relationship between companies and products. I have an intermediate table, Company_Product, that relates them.
Each company/product combination has a unique SKU. For example Acme widgets have SKU 123, but Omega widgets have SKU 456. I added the SKU as a field in the Company_Product intermediate table.
EF generated a model with a 1:* relationship between the company and Company_Product tables, and a 1:* relationship between the product and Company_Product tables. I really want a : relationship between company and product. But, most importantly, there's no way to access the SKU directly from the model.
Do I need to put the SKU in its own table and write a join, or is there a better way?
I just tested this in a new VS2010 project (EFv4) to be sure, and here's what I found:
When your associative table in the middle (Company_Product) has ONLY the 2 foreign keys to the other tables (CompanyID and ProductID), then adding all 3 tables to the designer ends up modeling the many to many relationship. It doesn't even generate a class for the Company_Product table. Each Company has a Products collection, and each Product has a Companies collection.
However, if your associative table (Company_Product) has other fields (such as SKU, it's own Primary Key, or other descriptive fields like dates, descriptions, etc), then the EF modeler will create a separate class, and it does what you've already seen.
Having the class in the middle with 1:* relationships out to Company and Product is not a bad thing, and you can still get the data you want with some easy queries.
// Get all products for Company with ID = 1
var q =
from compProd in context.Company_Product
where compProd.CompanyID == 1
select compProd.Product;
True, it's not as easy to just navigate the relationships of the model, when you already have your entity objects loaded, for instance, but that's what a data layer is for. Encapsulate the queries that get the data you want. If you really want to get rid of that middle Company_Product class, and have the many-to-many directly represented in the class model, then you'll have to strip down the Company_Product table to contain only the 2 foreign keys, and get rid of the SKU.
Actually, I shouldn't say you HAVE to do that...you might be able to do some edits in the designer and set it up this way anyway. I'll give it a try and report back.
UPDATE
Keeping the SKU in the Company_Product table (meaning my EF model had 3 classes, not 2; it created the Company_Payload class, with a 1:* to the other 2 tables), I tried to add an association directly between Company and Product. The steps I followed were:
Right click on the Company class in the designer
Add > Association
Set "End" on the left to be Company (it should be already)
Set "End" on the right to Product
Change both multiplicities to "* (Many)"
The navigation properties should be named "Products" and "Companies"
Hit OK.
Right Click on the association in the model > click "Table Mapping"
Under "Add a table or view" select "Company_Product"
Map Company -> ID (on left) to CompanyID (on right)
Map Product -> ID (on left) to ProductID (on right)
But, it doesn't work. It gives this error:
Error 3025: Problem in mapping fragments starting at line 175:Must specify mapping for all key properties (Company_Product.SKU) of table Company_Product.
So that particular association is invalid, because it uses Company_Product as the table, but doesn't map the SKU field to anything.
Also, while I was researching this, I came across this "Best Practice" tidbit from the book Entity Framework 4.0 Recipies (note that for an association table with extra fields, besides to 2 FKs, they refer to the extra fields as the "payload". In your case, SKU is the payload in Company_Product).
Best Practice
Unfortunately, a project
that starts out with several,
payload-free, many-to-many
relationships often ends up with
several, payload-rich, many-to-many
relationships. Refactoring a model,
especially late in the development
cycle, to accommodate payloads in the
many-to-many relationships can be
tedious. Not only are additional
entities introduced, but the queries
and navigation patterns through the
relationships change as well. Some
developers argue that every
many-to-many relationship should start
off with some payload, typically a
synthetic key, so the inevitable
addition of more payload has
significantly less impact on the
project.
So here's the best practice.
If you have a payload-free,
many-to-many relationship and you
think there is some chance that it may
change over time to include a payload,
start with an extra identity column in
the link table. When you import the
tables into your model, you will get
two one-to-many relationships, which
means the code you write and the model
you have will be ready for any number
of additional payload columns that
come along as the project matures. The
cost of an additional integer identity
column is usually a pretty small price
to pay to keep the model more
flexible.
(From Chapter 2. Entity Data Modeling Fundamentals, 2.4. Modeling a Many-to-Many Relationship with a Payload)
Sounds like good advice. Especially since you already have a payload (SKU).
I would just like to add the following to Samuel's answer:
If you want to directly query from one side of a many-to-many relationship (with payload) to the other, you can use the following code (using the same example):
Company c = context.Companies.First();
IQueryable<Product> products = c.Company_Products.Select(cp => cp.Product);
The products variable would then be all Product records associated with the Company c record. If you would like to include the SKU for each of the products, you could use an anonymous class like so:
var productsWithSKU = c.Company_Products.Select(cp => new {
ProductID = cp.Product.ID,
Name = cp.Product.Name,
Price = cp.Product.Price,
SKU = cp.SKU
});
foreach (var
You can encapsulate the first query in a read-only property for simplicity like so:
public partial class Company
{
public property IQueryable<Product> Products
{
get { return Company_Products.Select(cp => cp.Product); }
}
}
You can't do that with the query that includes the SKU because you can't return anonymous types. You would have to have a definite class, which would typically be done by either adding a non-mapped property to the Product class or creating another class that inherits from Product that would add an SKU property. If you use an inherited class though, you will not be able to make changes to it and have it managed by EF - it would only be useful for display purposes.
Cheers. :)
I have a many-to-many relationship between 2 entities in Entity Framework, like here . So, Employees and Projects. At one point, I would like to insert some Projects to a Employees entity in a specific order. By conserving the order I would like to know which was the first preference of the Employees for a Projects entity. The thing is that although I order the Student.Projectslist in the way I like before the insert, when selecting Employees.Projects.FirstOrDefault(), the entities are ordered after the ProjectsId and I don't get the first element I inserted. How can I conserve the order I want?
O course, I could make a new field PreferredProjects and save the other Projects in a random order, since only the preferred one is important for me. But this is not an option, being given the context of the current project's software design.
Thank you in advance...
It sounds like you simply want to have sorted child collection results when you do a query, rather than take full control of the insert order.
You can achieve that using the techniques described in Tip 1 of my tips series.
Hope this helps.
Alex
Program Manager Entity Framework Team.
Unfortunately, there is no easy solution. I have the same problem. The only solution is to save after adding each child item (project). This is the only way to save the order without using a new field column to sort input.
Try Employees.Projects.OrderBy(x => x).FirstOrDefault()