Using Entity Framework 4.1 Code First I have two objects with a many-to-many relationship:
public class Article
{
public int ID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public ICollection<Tag> Tags { get; set; }
}
public class Tag
{
[Key]
public string UrlSlug { get; set; }
public string Name { get; set; }
public ICollection<Article> Articles { get; set; }
}
I want to count the most common Tags applied to Articles. How do I do this in LINQ?
I tried the below code which only ordered the Tags:
var tagsFromDb = db.Tags
.GroupBy(q => q.UrlSlug)
.OrderByDescending(gp => gp.Count())
.Select(g => g.FirstOrDefault());
If I understand right you have a many-to-many relationship between articles and tags but the navigation property on the tag side is not exposed (there is no ICollection<Article> in Tag). I think in this case you have to start from the articles to get all used tags along with the information how often they are used in the articles:
var tagQuery = db.Articles
.SelectMany(a => a.Tags) // flat enumeration of all used tags
.GroupBy(t => t, (k, g) => new // k = key = Tag, g = group of same tags
{
Tag = k, // the tag
Count = g.Count() // how often does it occur in the articles
})
.OrderByDescending(g => g.Count); // most used tags first
If you want all tags sorted descending, call
var tagList = tagQuery.ToList();
If you just want the tag which is most often used in the articles, call
var mostUsedTag = tagQuery.FirstOrDefault();
In both cases the result is a collection/single object of an anonymous type which has the Tag and Count as members. If you just want the tag(s) and are not interested in the Count anymore you can project before apply ToList or FirstOrDefault: tagQuery.Select(anonymous => anonymous.Tag).....
Edit
Just saw the Edit in your question. Now, when you have an Article collection in the Tag entity it is easier:
var tagQuery = db.Tags
.Select(t => new
{
Tag = t, // the Tag
Count = t.Articles.Count() // just count the number of artices
// the tag is used in
})
.OrderByDescending(x => x.Count); // most used tags first
Related
Hope you're doing well,
I was trying to optimize my reads with entity framework, where I arrived at a position, where I get a record from database by id, and I want to include a one-to-many related list, but I don't want to get all data of the list, just a few, so I want to kind of paginate it.
I want to do this process as long as data is in IQueryable state, I don't want to load all data of list in memory and that paginate it as enumerable.
Let's say the query is like below:
var author = await _dbContext.Authors.Where(x => x.Id == id)
.Include(x => x.Books) // <-- paginate this !!??
.FirstOrDefaultAsync();
Entities represent Data state. Pagination and presentation concerns are View state. Entity Framework can help bridge that gap, but it does so by enabling projection so that you can build View state from Data state. Don't pass entities to views, instead build and pass ViewModels to represent the data in accordance to translations and limitations you want for the view.
For instance if you want to pass Author details with their 5 most recent books:
public class AuthorViewModel
{
public int Id { get; set; }
public string Name { get; set; }
// Any other relevant fields...
public ICollection<BookViewModel> RecentBooks = new List<BookViewModel>();
}
public class BookViewModel
{
public int Id { get; set; }
public string Name { get; set; }
public DateTime PublishedDate { get; set; }
// Any other relevant fields...
}
var author = await _dbContext.Authors
.Where(x => x.Id == id)
.Select( x => new AuthorViewModel
{
Id = x.Id,
Name = x.Name,
RecentBooks = x.Books
.OrderByDescending(b => b.PublishedDate)
.Select(b => new BookViewModel
{
Id = b.Id,
Name = b.Name,
PublishedDate = b.PublishedDate
}).Take(5)
.ToList()
}).SingleOrDefault();
This gives you the benefit of structuring the data how you want to present it, while generating efficient queries against the database. You can configure tools like Automapper to perform this kind of mapping to use it's ProjectTo<AuthorViewModel>() as a more succinct alternative.
I have a List<Product> contains properties of Bikes (Name, ProductName, Color, List Price). I'm struggling to figure out how to write a search function using LINQ. I'd like to find a name of Bike. Any suggest will be help me some ways.
Imagine that your name is taken from a variable called nameToSearch.
This is if you want to get the Product.
string nameToSearch = "BikeName";
List<Product> list = bikes.Where(x => x.Name == nameToSearch).ToList();
I assume you have the following Product class:
public class Product
{
public String Name { get; set; }
public String ProductName { get; set; }
public String Color { get; set; }
public String List { get; set; }
public String Price { get; set; }
}
You also mentioned you have your data in a List<Product>. I will give a demo name for it:
List<Product> myProductList = GetProductList();
// Where GetProductList() will create a new List<Product> and populate it.
String bikeNameFilter = GetNameFilter();
// You can chnage this by the string you want for filtering.
You can use the following to get your data:
List<Product> myFilteredProductList = (from p in myProductList
where p.Name = bikeNameFilter
select p;
).ToList()
Obviously you can change the filter you want to use to another property of your product. Finally to get the actual name, you can loop through the list you just got:
foreach (var p in myFilteredProductList)
{
Console.WriteLine(p.ProductName);
// Use this value wherever you want.
}
Take a look at a nuget package I have created
http://www.nuget.org/packages/NinjaNye.SearchExtensions
This will enable the following (and more) which will return results where the search term appears in any of the properties specified
var result = products.Search("searchTerm", p => p.Name, p => p.ProductName);
Performing a search against all string properties can be done as follows:
var result = products.Search("searchTerm");
Alternatively, you can perform an AND search where the search term exists in a set of properties as follows:
string searchTerm = "searchTerm";
var result = products.Search(searchTerm, p => p.Name)
.Search(searchTerm, p => p.ProductName);
For more information take a look at the projects GitHub page or my blog posts
UPDATE: don't forget the using directive...
using NinjaNye.SearchExtensions
I'm trying to update a Customer in my database using ASP.NET Web API and Entity Framework 5 code-first, but it's not working. My entities look like this:
public class CustomerModel
{
public int Id { get; set; }
public string Name { get; set; }
// More fields
public ICollection<CustomerTypeModel> CustomerTypes { get; set; }
}
public class CustomerTypeModel
{
public int Id { get; set; }
public string Type { get; set; }
[JsonIgnore]
public ICollection<CustomerModel> Customers { get; set; }
}
Nothing all that special. I've built a web interface where users can add a customer by supplying the name and checking one or more customer types. When hitting the submit button, the data is sent to my Web API method:
public void Put([FromBody]CustomerModel customer)
{
using (var context = new MyContext())
{
context.Customers.Attach(customer);
context.Entry(customer).State = EntityState.Modified;
context.SaveChanges();
}
}
This updates the customer fields, but the related customer types are ignored. The incoming customer object does contain a list of CustomerTypes it should be associated with:
[0] => { Id: 1, Type: "Finance", Customers: Null },
[1] => { Id: 2, Type: "Insurance", Customers: Null }
[2] => { Id: 3, Type: "Electronics", Customers: Null }
But instead of looking at this list and adding/removing associated entities, EF just ignores it. New associations are ignored and existing associations remain even if they should be deleted.
I had a similar problem when inserting a customer into the database, this was fixed when I adjusted the state of these entities to EntityState.Unchanged. Naturally, I tried to apply this same magic fix in my update scenario:
public void Put([FromBody]CustomerModel customer)
{
using (var context = new MyContext())
{
foreach (var customertype in customer.CustomerTypes)
{
context.Entry(customertype).State = EntityState.Unchanged;
}
context.Customers.Attach(customer);
context.Entry(customer).State = EntityState.Modified;
context.SaveChanges();
}
}
But EF keeps displaying the same behavior.
Any ideas on how to fix this? Or should I really just do a manual clear to the list of CustomerTypes and then manually add them?
Thanks in advance.
JP
This is not really solvable by only setting entity states. You must load the customer from the database first including all its current types and then remove types from or add types to the loaded customer according to the updated types collection of the posted customer. Change tracking will do the rest to delete entries from the join table or insert new entries:
public void Put([FromBody]CustomerModel customer)
{
using (var context = new MyContext())
{
var customerInDb = context.Customers.Include(c => c.CustomerTypes)
.Single(c => c.Id == customer.Id);
// Updates the Name property
context.Entry(customerInDb).CurrentValues.SetValues(customer);
// Remove types
foreach (var typeInDb in customerInDb.CustomerTypes.ToList())
if (!customer.CustomerTypes.Any(t => t.Id == typeInDb.Id))
customerInDb.CustomerTypes.Remove(typeInDb);
// Add new types
foreach (var type in customer.CustomerTypes)
if (!customerInDb.CustomerTypes.Any(t => t.Id == type.Id))
{
context.CustomerTypes.Attach(type);
customerInDb.CustomerTypes.Add(type);
}
context.SaveChanges();
}
}
A cleaner solution would be:
public void Put([FromBody]CustomerModel customer)
{
using (var context = new MyContext())
{
var customerInDb = context.Customers.Include(c => c.CustomerTypes)
.Single(c => c.Id == customer.Id);
// Updates the Name property
context.Entry(customerInDb).CurrentValues.SetValues(customer);
// Remove types
customer.CustomerTypes.Clear();
// Add new types
foreach (var type in customer.CustomerTypes)
{
context.CustomerTypes.Attach(type);
customerInDb.CustomerTypes.Add(type);
}
context.SaveChanges();
}
}
I have a typical many-to-many relationship with these 3 tables
[Post] (
[PostId] int, (PK)
[Content] nvarchar(max)
...
)
[Tag] (
[TagId] int, (PK)
[Name] nvarchar
...
)
[TagPost] (
[TagId] int, (PK, FK)
[PostId] int (PK, FK)
)
And, TagId and PostId are the PK and FK set on the tables accordingly etc. Then I have these classes and mapping in c#
public class Post {
public Post()
{
this.Tags = new HashSet<Tag>();
}
[Key]
public int PostId { get; set; }
...
public virtual ICollection<Tag> Tags { get; private set; }
}
public class Tag {
public Tag()
{
this.Posts = new HashSet<Post>();
}
[Key]
public int TagId { get; set; }
...
public virtual ICollection<Post> Posts { get; private set; }
}
internal class MyDbContext : DbContext
{
public DbSet<Post> Posts { get; set; }
public DbSet<Tag> Tags { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<Post>().ToTable("Post");
modelBuilder.Entity<Tag>().ToTable("Tag");
modelBuilder.Entity<Post>()
.HasMany(x => x.Tags)
.WithMany(x => x.Posts)
.Map(x =>
{
x.ToTable("TagPost");
x.MapLeftKey("PostId");
x.MapRightKey("TagId");
});
}
Then I have this code to query them
var list = (from p in ctx.Posts.Include(p => p.Tags)
from t in p.Tags
where ... // some of my filter conditions
select p).ToList();
This join does return the posts I was looking for, however the returned posts don't their associated tags filled in even though I have the Include there. Could someone help point out what I'm missing so that I could have the tags also return with the posts?
Thanks a lot.
The double from is a manual Join which causes the Include to be ignored as mentioned here and here. Include is also ignored for other LINQ methods like grouping and projections.
Relationship fixup generally does not work for many-to-many relationships, only for relationships which have at least one single reference at one of the ends - one-to-many or one-to-one. If you project the Posts and related Tags into another type (anonymous or named) the data will be loaded correctly but because the relationship is many-to-many EF won't create the relationship in memory automatically so that the post.Tags collection will stay empty.
To get the Include working you must remove the second from from your query and apply the where clause directly to the Post entity parameter, for example like so:
var list = (from p in ctx.Posts.Include(p => p.Tags)
where p.Tags.Any(t => t.TagId == 1)
select p).ToList();
The filter by a Tag property is specified in the expression passed into .Any which is an expression with a Tag (t) as parameter.
try selecting everything into an anonymous object (something like this)
var list = (
from p in ctx.Posts
from t in p.Tags
where ... // some of my filter conditions
select new {
Posts = p,
Tags = p.Tags
})
.ToList();
Based on the feedback to my initial answer and the fact that EF can find the related entities but it is failing to populate the Tags collection I believe the issue lies in the definition of the Tags entity in the Post class.
Try removing the Hashset<> initialiser from the constructors and private from the set declaration:
public virtual ICollection<Tag> Tags { get; set; }
How do I created the appropriate AbstractIndexCreationTask for the following scenario?
For a scenario of multiple blogs, how do I get the tags from specific blogs and the tag-count for these?
Members of interest for data-structure stored in RavenDB:
public class BlogPost {
public string BlogKey { get; set; }
public IEnumerable<string> Tags { get; set; }
/* ... */
}
The method I need to implement has the following signature:
public Dictionary<string, int> GetTagsByBlogs(string tag, params string[] blogKeys)
In normal LINQ I would write something like this:
var tags = from post in blogPosts
from tag in post.Tags
where blogKeys.Contains(post.BlogKey)
group tag by tag into g
select new {
Tag = g.Key,
Count = g.Count(),
};
But neither SelectMany or GroupBy are supported in RavenDB. I've tried different combinations for a map-reduce solution, but I can't figure out how to do this since the map and the reduce differ in data-structure.
How to create a tag cloud is described in the knowledge base of RavenDB.
In your case, you have to include the BlogKey in the index, especially in the group by clause:
public class Tags_Count : AbstractIndexCreationTask<BlogPost, Tags_Count.ReduceResult>
{
public class ReduceResult
{
public string BlogKey { get; set; }
public string Name { get; set; }
public int Count { get; set; }
}
public Tags_Count()
{
Map = posts => from post in posts
from tag in post.Tags
select new {
BlogKey = post.BlogKey,
Name = tag.ToString().ToLower(),
Count = 1
};
Reduce = results => from tagCount in results
group tagCount by new {
tagCount.BlogKey,
tagCount.Name } into g
select new {
BlogKey = g.Key.BlogKey,
Name = g.Key.Name,
Count = g.Sum(x => x.Count)
};
Sort(result => result.Count, SortOptions.Int);
}
}
Then query that index with the desired BlogKey:
var result = session.Query<Tags_Count.ReduceResult, Tags_Count>()
.Where(x => x.BlogKey = myBlogKey)
.OrderByDescending(x => x.Count)
.ToArray();
If you need to query for multiple blogs, you can try this query:
var tagsByBlogs = session.Query<Tags_Count.ReduceResult, Tags_Count>()
.Where(x => x.BlogKey.In<string>(blogKeys))
.OrderByDescending(x => x.Count)
.ToArray();
AFAIK that is as far as you can get with an index. You still have to aggregate the results on the client side as you did in your original question, except that you can skip the filtering on blogKeys:
var tags = from tag in tagsByBlogs
group tag by Name into g
select new {
Tag = g.Key,
Count = g.Count(),
};
Take a look at faceted search, you can specify the criteria at query time, like so:
var facetResults = s.Query<BlogPost>("BlogIndex")
.Where(x => x.BlogKey == "1" || x.BlogKey == "5" ...)
.ToFacets("facets/BlogFacets");
Then the grouping (and counts) is done on all the results that match the where clause.
Update You'll need an index that looks something like this:
from post in blogPosts
from tag in post.Tags
select new
{
post.BlogKey
Tag = tag
}