Need help with a orm request.
There are 2 tables. Author and Book (names for example, so you don't need to look at the logic), linked through FK.
class Book(models.Models):
title = models.Charfield(...)
class Author(models.Model):
book = models.ForeignKey(Book)
Need to group the authors by the book and go through them in a loop. The question is how to select only the last 50 authors of each book.
I can write this:
for book in Book.all()
for author in book.author_set.all()[:50]:
....
But this is not an optimal solution.
Bad answer, I didn't realize that slicing implies a new query. My bad, don't look further down.
To optimize your query, you need to use the prefetch_related() method:
prefetch_related()
Returns a QuerySet
that will automatically retrieve, in a single batch, related objects
for each of the specified lookups.
This has a similar purpose to select_related, in that both are
designed to stop the deluge of database queries that is caused by
accessing related objects, but the strategy is quite different.
select_related works by creating an SQL join and including the fields
of the related object in the SELECT statement. For this reason,
select_related gets the related objects in the same database query.
However, to avoid the much larger result set that would result from
joining across a ‘many’ relationship, select_related is limited to
single-valued relationships - foreign key and one-to-one.
prefetch_related, on the other hand, does a separate lookup for each
relationship, and does the ‘joining’ in Python. This allows it to
prefetch many-to-many and many-to-one objects, which cannot be done
using select_related, in addition to the foreign key and one-to-one
relationships that are supported by select_related.
for book in Book.all().prefetch_related()
for author in book.author_set.all()[:50]
You also need to order your book.author_set queryset to make sure you get the latest entries.
When talking about n to m relationships in a database, an intersection table is often mentioned.
E.g. there are authours and books, an author can have a relationship to many books and vice versa.
But: How can I model the relationship, that the authors can also have relationships among them and books, too?
E.g.
- Author A has written Book 1 and Book 2, and Author A is a friend of Author B.
- Book 1 is related to the same genre as Book 2.
Do I have multiple Intersection tables then?
E.g. An Author - Book Relationship table, an Author -Author "Friend"-Relationship table and a Book-Book "Genre" Relationship?
I am a beginner concerning databases and SQL...
Thank you for your help,
Anne
To create a simple 1:n relation-ship, it's enough to have one FOREIGN KEY-column to specify the related record.
But whenever you want to model a m:n relationship, or you might need to add additional information to a 1:n relationship, you'll need a mapping table.
mapping tables with 1:n relations: Imagine the case, that a book can have exactly one current reader. If you'd use a FK CurrentReaderID in the Book table, you would not be able to find previous readers once you've changed the FK-value.
mapping tables with m:n relations: Think of the relation as an object on its own with its own specific detail data (when, validTo, createdBy...)
And yes: In a well designed database you'll find many tables with a wide range of different relations / mappings among them.
Yes, this is fairly standard. If books can have multiple genres, a Book-Genre table containing Genre IDs from one side and Book Ids from the other is perfectly appropriate. The Author-Friend relation is a bit trickier as the intersection table will join to the Author table twice (once for each author in the author-friend relation), but is essentially the same.
As you've identified, these are the same as a Book-Author table that links books to their authors on a n:m basis.
Look for student-teacher-course examples as they are similar and often part of an n:m explanation.
Let's say I have a two tables - book and author. A book can have many authors and an author can have many books, which means they are in a many to many relationship and I'll need a third table to implement that.
I really want to avoid this, as I am a beginner and this makes the queries more complicated for me.
So my question is : what would be the best way to implement this type of relationship excluding many-to-many?
I was thinking just to put a foreign key in each table so I have an authoridFK in my books table and bookIdFK in my authors table. But I am not sure what will type of a relationship would that be, will it me correct and how it will look on an ERD?
Can someone clarify these to me?
The 'best', and most standard way is to use a third table to maintain the relationship.
As for query complexity, you can either look into using JOIN in your queries or getting an ORM which handles these type of relationships for you.
An example join query for the given context which will return a list of all authors and all books they have contributed to would look like this:
select a.AuthorName, b.BookName
from author a
join AuthorBookMapping m on m.AuthorID = a.AuthorID
join Book b on m.BookID = b.BookID
The intermediate table is the only correct way to build your relation.
This logic does not complicate the structure, but on the opposite, it allows to comply with good practices:
each data item must be located in a unique place in the database
each record (object) must have a unique primary key.
This way you can both link authors to their books, possibly assign several authors to the same book, and of course several books to the same author.
Advanced CRUD systems also allow you to display the author(s) in the book list and edit these relationships.
Here's a tutorial about relationships in a MySQL database
It explains the main things about direct / indirect relationships, and identifying / non-identifying relationships
I have an app where users can log comments related to any specific entity that the system has and I wanted to know if there is a "best practice" way to handle the design of the db for this kind of feature.
For example: Three current entities (tables) deal are Question, Documentation, ReferenceMaterial (self explanatory what each hold). The user can leave a comment on any one of those particular items and all comments are simply a varchar field, user id, and date of comment. EDIT: Comments can also belong to more than one entity. For example, all Question entities belong to a Quiz or Test entity. Each of those (Quiz and Test), can also have comments associated with themselves. So you could run a report to see all comments left for a test and easily just query the Comment table for every record with that test foreign key, or you could limit your query to just the comments left for questions in that test, or a particular question itself. It offered a lot of flexibility END EDIT
Right now the way that I hvae this is one Comment table with a foreign key relationship with each of the other entity tables (i.e. fkQuestion, fkDocumentation, fkReferenceMaterial, etc). So all comments in the system are stored in this table and based on what page the user is on, I conduct the join to that particular entity's records.
Is there a best practice way of doing this?
Thanks in advance for any help.
I am learning about databases and SQL for the first time. In the text I'm reading (Oracle 11g: SQL by Joan Casteel), it says that "many-to-many relationships can't exist in a relational database." I understand that we are to avoid them, and I understand how to create a bridging entity to eliminate them, but I am trying to fully understand the statement "can't exist."
Is it actually physically impossible to have a many-to-many relationship represented?
Or is it just very inefficient since it leads to a lot of data duplication?
It seems to me to be the latter case, and the bridging entity minimizes the duplicated data. But maybe I'm missing something? I haven't found a concrete reason (or better yet an example) that explains why to avoid the many-to-many relationship, either in the text or anywhere else I've searched. I've been searching all day and only finding the same information repeated: "don't do it, and use a bridging entity instead." But I like to ask why. :-)
Thanks!
Think about a simple relationship like the one between Authors and Books. An author can write many books. A book could have many authors. Now, without a bridge table to resolve the many-to-many relationship, what would the alternative be? You'd have to add multiple Author_ID columns to the Books table, one for each author. But how many do you add? 2? 3? 10? However many you choose, you'll probably end up with a lot of sparse rows where many of the Author_ID values are NULL and there's a good chance that you'll run across a case where you need "just one more." So then you're either constantly modifying the schema to try to accommodate or you're imposing some artificial restriction ("no book can have more than 3 authors") to force things to fit.
A true many-to-many relationship involving two tables is impossible to create in a relational database. I believe that is what they refer to when they say that it can't exist. In order to implement a many to many you need an intermediary table with basically 3 fields, an ID, an id attached to the first table and an id atached to the second table.
The reason for not wanting many-to-many relationships, is like you said they are incredibly inefficient and managing all the records tied to each side of the relationship can be tough, for instance if you delete a record on one side what happens to the records in the relational table and the table on the other side? Cascading deletes is a slippery slope, at least in my opinion.
Normally (pun intended) you would use a link table to establish many-to-many
Like described by Joe Stefanelli, let's say you had Authors and Books
SELECT * from Author
SELECT * from Books
you would create a JOIN table called AuthorBooks
Then,
SELECT *
FROM Author a
JOIN AuthorBooks ab
on a.AuthorId = ab.AuthorId
JOIN Books b
on ab.BookId = b.BookId
hope that helps.
it says that "many-to-many relationships can't exist in a relational database."
I suspect the author is just being controversial. Technically, in the SQL language, there is no means to explicitly declare a M-M relationship. It is an emergent result of declaring multiple 1-M relations to the table. However, it is a common approach to achieve the result of a M-M relationship and it is absolutely used frequently in databases designed on relational database management systems.
I haven't found a concrete reason (or better yet an example) that explains why to avoid the many-to-many relationship,
They should be used where they are appropriate to be used would be a more accurate way of saying this. There are times, such as the books and authors example given by Joe Stafanelli, where any other solution would be inefficient and introduce other data integrity problems. However, M-M relationships are more complicated to use. They add more work on the part of the GUI designer. Thus, they should only be used where it makes sense to use them. If you are highly confident that one entity should never be associated with more than one of some other entity, then by all means restrict it to a 1-M. For example, if you were tracking the status of a shipment, each shipment can have only a single status at any given time. It would over complicate the design and not make logical sense to allow a shipment to have multiple statuses.
Of course they can (and do) exist. That sounds to me like a soapbox statement. They are required for a great many business applications.
Done properly, they are not inefficient and do not have duplicate data either.
Take a look at FaceBook. How many many-to-many relationships exist between friends and friends of friends? That is a well-defined business need.
The statement that "many-to-many relationships can't exist in a relational database." is patently false.
Many-to-many relationships are in fact very useful, and also common. For example, consider a contact management system which allows you to put people in groups. One person can be in many groups, and each group can have many members.
Representation of these relations requires an extra table--perhaps that's what your book is really saying? In the example I just gave, you'd have a Person table (id, name, address etc) and a Group table (id, group name, etc). Neither contains information about who's in which group; to do that you have a third table (call it PersonGroup) in which each record contains a Person ID and a Group ID--that record represents the relation between the person and the group.
Need to find the members of a group? Your query might look like this (for the group with ID=1):
SELECT Person.firstName, Person.lastName
FROM Person JOIN PersonGroup JOIN Group
ON (PersonGroup.GroupID = 1 AND PersonGroup.PersonID = Person.ID);
It is correct. The Many to Many relationship is broken down into several One to Many relationships. So essentially, NO many to many relationship exists!
Well, of course M-M relationship does exist in relational databases and they also have capability of handling at some level through bridging tables, however as the degree of M-M relationship increases it also increases complexity which results in slow R-W cycles and latency.
It is recommended to avoid such complex M-M relationships in a Relational Database. Graph Databases are the best alternative and good at handling Many to Many relationship between objects and that's why social networking sites uses Graph databases for handling M-M relationship between User and Friends, Users and Events etc.
Let's invent a fictional relationship (many to many relationship) between books and sales table. Suppose you are buying books and for each book you buy needs to generate an invoice number for that book. Suppose also that the invoice number for a book can represent multiple sales to the same customer (not in reality but let's assume). We have a many to many relationship between books and sales entities.
Now if that's the case, how can we get information about only 1 book given that we have purchased 3 books since all books would in theory have the same invoice number? That introduces the main problem of using a many to many relationship I guess. Now if we add a bridging entity between Books and sales such that each book sold have only 1 invoice number, no matter how many books are purchases we can still correctly identify each books.
In a many-to-many relationship there is obvious redundancy as well as insert, update and delete anomaly which should be eliminated by converting it to 2 one-to-many relationship via a bridge table.
M:N relationships should not exist in database design. They are extremely inefficient and do not make for functional databases. Two tables (entities) with a many-to-many relationship (aircraft, airport; teacher, student) cannot both be children of each other, there would be no where to put foreign keys without an intersecting table. aircraft-> flight <- airport; teacher <- class -> student.
An intersection table provides a place for an entity that is dependent on two other tables, for example, a grade needs both a class and a student, a flight needs both an aircraft and an airport. Many-to-many relationships conceal data. Intersection tables reveal this data and create one-to-many relationships that can be more easily understood and worked with. So, the question arises, what table should the flight be in--aircraft or airport. Neither, they should be foreign keys in the intersection table, Flight.