A general parse.com relationship question really - using swift. I have a blogging app, for which there is a blog class, and a user class (along with a few others!) the blog class stores the associated user ID in a field for simplicity. Can I use includekey (or something similar) in a pfquery for the following;
firstly retrieve specific (or all) blog entries that match a criteria.
for each matching blog entry, check a field in the related user class for an option before returning the JSON list of entries
I suppose, sort of a subquery really, but wanted the whole thing to work in one pfquery if possible.
thanks!
Yes, you can do this with a relational query. The user stored in blog should be a pointer.
First create a query for the field in the user class, i.e.
userQuery.whereKey("age", greaterThan: 30)
(Do not execute this query)
Then, when adding constraints to your blog query, add
blogQuery.whereKey("user", matchesQuery: userQuery)
Related
I am using go and pq to interface with my postgres database.
I have a simple user table which has basic fields. Id, name, type. My auxillary table, admin inherits from user and adds it's own field panel, and another one that is owner and adds owner. Whether that be using table inheritance, or a supporting table.
My question is if I hit and endpoint that points to user/1 at this point I don't know what type of user this person is yet here. I know we can use jwts and other ways to provide this from the front end. I'm more curious about if there is a way to figure out the user and it's type and query the additional fields in one query?
Ie. I hit the endpoint I would Select from users, get the type, then use that type to get the additional fields. So I would effectively be doing two queries on two tables to get the complete data. Is there a better solution of doing this? Is there some optimizations I could do.
I have a two aggregate roots in my domain, and therefore two repositories. We'll call them BookRepository, and AuthorRepository, for the sake of example.
I'm designing an MVC application, and one page has to display a table containing a list of authors, with each row showing the author's personal details. At the end of each row is a small button that can be clicked to expand the row and show a child table detailing the author's published books.
When the page loads, some ajax is executed to retrieve the Author details from an API controller and display the data in the table. Each property in an Author object maps almost directly to a column, with one exception, and this is where I'm having my problem. I want the button at the end of each row to be disabled, if and only if the author has no published books. This means that a boolean has to returned with each Author record, indicating if they have any published books.
My book repository has a couple of methods like this:
public IEnumerable<Book> GetBooksForAuthor(int authorId);
public bool AnyBooksForAuthor(int authorId);
and my Book class has a property called AuthorId, so I can retrieve a book's author by calling
authorRepository.GetById(book.AuthorId);
My problem is that in order to create a row for my aforementioned table, I need to create it like this:
IEnumerable<Author> authors = authorRepository.GetAll();
foreach (Author author in authors)
{
yield return new AuthorTableRow
{
Name = author.Name,
Age = author.Age,
Location = author.PlaceOfResidence.Name,
HasBooks = this.bookRepository.AnyBooksForAuthor(author.Id)
};
}
The above code seems correct, but there's a fairly heft performance penalty in calling this.bookRepository.AnyBooksForAuthor(author.Id) for every single author, because it performs a database call each time.
Ideally, I suppose I would want an AuthorTableRowRepository which could perform something like the following:
public IEnumerable<AuthorTableRow> GetAll()
{
return from a in this.dbContext.Authors
select new AuthorTableRow
{
Name = a.Name,
Age = a.Age,
Location a.PlaceOfResidence.Name
HasBooks = a.Books.Any()
});
}
I'm hesitant to put this in place for these reasons :
AuthorTableRowRepository is a repository of AuthorTableRows, but AuthorTable row is not a domain object, nor an aggregate root, and therefore should not have its own repository.
As Author and Book are both aggregate roots, I removed the "Books" property from the Author entity, because I wanted the only way to retrieve books to be via the BookRepository. This makes HasBooks = a.Books.Any() impossible. I am unsure whether I am imposing my own misguided best practice here though. It seems wrong to obtain Books by obtaining an Author via the AuthorRepository and then going through its Books property, and vice versa in obtaining an Author via a property on a Book object. Crossing aggregate root boundaries would be the way I'd term it, I suppose?
How would other people solve this? Are my concerns unfounded? I am mostly concerned about the (what should be a) performance hit in the first method, but I want to adhere to best practice with the Repository pattern and DDD.
I would stick to the first approach, but try to optimize things in the bookrepository method. For instance, you can load this information all in one time, and use in-memory lookup to speed this up. Like this you would need 2 queries, and not 1 for each author.
The way I solved this in the end was to create an Entity from a view in the database. I named the entity 'AuthorSummary', and made an AuthorSummaryRepository that didn't contain any Add() methods, just retrieval methods.
I'm trying to develop a many-to-many relationship between tags (in the tags table) and items (in the items table) using a field of type integer[] on each item.
I know that Rails 4 (and Rails 3 via postgres_ext) has support for Postgres' arrays feature through the :array => true parameter, but I can't figure out how to combine them with Active Record associations.
Does has_many have an option for this? Is there a gem for this? Should I give up and just create a has_many :through relationship (though with the amount of relations I'm expecting this is probably unmanageable)?
At this point, there isn't a way to use relationships with arrays in Rails. Using the selected answer though, you will run into the N+1 select issue. Say you get your posts and then the tags for it on each post with "tags" method defined in the class. For each post you call the tags on, you will incur another database hit.
Hopefully, this will change in the future and we can get rid of the join table (especially given that Postgres 9.4 will include support for foreign keys in Arrays).
All you really need to do is
def tags
Tag.where(id: tag_ids)
end
def add_tag(tag)
self.tag_ids += [tag.id] unless tag_ids.include?(tag.id)
end
At least that's what I do at the moment. I do some pretty cool stuff with hashes (hstore) as well with permissions. One way of handling tags is to create the has_many through and persist the tags in a string array column as they are added for convenience and performance (not having to query the 2 related tables just to get the names out). I you don't necessarily have to use active record to do cool stuff with the database.
I want to implement content tagging using MongoDB. In a relational database, the best approach would be to have a many-to-many relation between the content (say, "products") and tags tables. But what is best approach with NoSQL databases?
Would it be better to put every tag in a tags array of the "content" document, or put references to tags in a string?
In most cases where you have a n:m relation in MongoDB, you should use embedding instead of referencing. So I would recommend you to have an array "tags" in each product with the tag names. I assume that looking at a single product will be the most frequent use-case in your system. This design will allow you to show the user a product with a list of tag names with a single database query.
When you need some additional meta-data about the tags which you don't want to bind to a product (like a long-text description of a tag), you could create an additional tags collection, where the name field gets an unique index for fast lookup and avoiding duplicates. When the user clicks on or hovers over a tag name, you can use an additional query to get the tag details.
A problematic case in this design is the situation when you want to delete or rename a tag. Then you have to edit every product which includes the tag. But because MongoDB doesn't know foreign keys with CASCADE ON DELETE like SQL databases, you will always have that problem when you have documents referencing one another.
Renaming tags could be made easier by storing objectIDs instead of names in the tag array of the product. But IDs have the disadvantage that they are useless for the user. You need to get the names of the tags to show a product page. That means that you have to request every single one from the tags collection, which requires an additional database query.
In my web app a user can assign muliple tags to products (like the tagging here on stackoverflow).
There are three tables: products, tags and products_tags to implement a many to many relationship.
My question is, how would you implement this with the Entity Framework (LINQ to SQL):
"Insert only a new tag in the tags table if it doesnt already exist there".
So before the insert i have to check first if a tag exists, whats the best way to accomplish this (best performance) ??
thanks for answers
Simple: The Tag should then be the user assigned key/PK of the entity/table.
If you have troubles synchronizing this with the database, I am sure there's something like (N)Hibernate's merge Method in EntityFramework.