I have a situation where i need a relation between a table and a pivot table of a many to many relation. I have no idea how to solve this properly, especially in Laravel. As my real code and scenario makes the question more complex than needed I will explain my question based on a simple and realistic scenario.
Asume I build an application where gamers can schedule when they play which game so frieds can decide to reservate some time to join the game. In this case i have a bunch of gamers and a bunch of games they can play. This means a game and gamer can have a relation to eachother even if they have no concrete time sceduled to play the game. After they have a relation with the games they own, they can add "time windows" to the relation.
I think this can be solved like this in the DB where the yellow tables are pivot tables:
I started with defining the models Gamer, Game and the relation between those 2:
Game.php
...
class Game extends Model
{
return $this->belongsTomany(Games::class, 'gamer_game', 'game_id', 'gamer_id');
}
...
Gamer.php
...
class Gamer extends Model
{
return $this->belongsTomany(Gamers::class, 'gamer_game', 'gamer_id', 'game_id');
}
...
This works perfect, i can add relations between games and gamers as many as i want and simply query them in Tinker like Gamer::first()->games or Games::first()->gamers
Now i want to extend this so they can add one or more availability to the relation. But now i run into a problem. When i do Game::first()->gamers[0]->pivot i don't get the ID of the pivot entry so i cant create the entry for the gamer_game_availability table. I could create a model of the pivot table but is this a nice approach?
I have no idea if its good practice at all to have a relation between pivot tables so i also thought about something like adding an availability_id to the gamer_game table but i'm not sure if its good practice to use 1 pivot table for a relation between 3 tables. And if its the way to go, how should i deal with situations where a gamer has a relation to a game but not scheduled a time window yet. Leave the availability_id emtpy (null) ?
Whats the best practice in situations like this and how can i implement it in Laravel?
Related
Sorry if this is a relatively easy problem to solve; I read the docs on inheritance and I'm still confused on how I would do this.
Let's say I have the parent table being car_model, which has the name of the car and some of it's features as the columns (e.g. car_name, car_description, car_year, etc). Basically a list of cars.
I have the child table being car_user, which has the column user_id.
Basically, I want to link a car to the car_user, so when I call
SELECT car_name FROM car_user WHERE user_id = "name", I could retrieve the car_name. I would need a linking component that links car_user to the car.
How would I do this?
I was thinking of doing something like having car_name column in car_user, so when I create a new data row in car_user, it could link the 2 together.
What's the best way to solve this problem?
Inheritance is something completely different. You should read about foreign keys and joins.
If one user drives only one car, but many users can drive same car, you need to build one-to-many -relation. Add car_name to your user table and JOIN using that field.
I'm using .Net 4.5, entity framework 5, database first. I have a junction (many-to-many) table in my database. For this example lets say the table is "StudentsCourses":
Students
-PkStudentId
-Name
Courses
-PkCourseId
-CourseNumber
StudentsCourses
-FkStudentId
-FkCourseId
This works just fine right now. The 'generate model from database' creates a Student entity with a navigation property to the Course entity. But here is where the trouble is:
I need to add another column to the StudentsCourses table. Lets just call this column "CourseYear". So our junction table would now look like this:
StudentsCourses
-FkStudentId
-FkCourseId
-CourseYear
So, I've added this column to the database and ran "Update Model from Database" on the edmx. I would expect to see an entity created for StudentCourses, with a navigation property to both Students and Courses. But no such entity is created. I still see the same two tables (Students & Courses) with the same navigation property as before.
I've done a lot of reading and researching, but haven't really come across a clear-cut answer. There is a wealth of information on code-first which I can't apply to my scenario. Is there a way to get what I'm after? Is it as simple as adding a PkId to the StudentCourses table? SQL Replication is preventing me from doing this. I would think the composite should suffice, but maybe EF needs a PK to do it's magic? I read a little bit about manually setting relationships, but could not find anything speaking to my particular situation. It could be that I am just missing a simple step in the process of updating the edmx from database. I've done this plenty of times when: adding new tables, adding columns, deleting columns, etc. I'm following the same steps as I always do, but maybe I need to do something different in this case?
Thanks ahead of time for any help. It is greatly appreciated. Please let me know if any more information would help.
From what I've gathered it appears as though EF will not generate a model for a table that doesn't have a Primary Key.
I'm a bit late for this, but you have the answer in this thread Updating Entity Framework Model after adding a field to a previous look up only table
As they say here, you have to delete the relationship between Students and Courses in the designer. Then update your model from the database, and make sure StudentsCourses table is checked in the Tables branch of the Add tab.
Just starting out with Entity Framework (Code First) and I have to say I am having a lot of problems with it when loading SQL data that is fairly complex. For example, let's say I have the following tables which stores which animals belongs to which regions in the world and the animal are also categorized.
Table: Region
Id: integer
Name string
Table AnimalCategory
Id integer
Name: string
RegionId: integer -- Refers back Region
Table Animal
Id integer
AnimalCategoryId integer -- Refers back AnimalCategory
Let's say I want to create a query with Entity Framework that would load all Animals for a specific region. The easiest thing to do is to create 3 Entities Region, AnimalCategory, and Animal and use LINQ to load the data.
But let's say I am not interested in loading any AnimalCategory information and define an Entity class just to represent AnimalCategory so that I can do the JOIN. How can I do this with Entity Framework? Even with many of its Mapping functions I still don't think this is possible.
In non Entity Framework solutions this is easy to accomplish by using INNER JOINs in SPs or inline SQL. So what are my options in Entity Framework? Shall I pollute my data model with these useless tables just so I can do a JOIN?
It's a matter of choice I guess. EF choose to support many-to-many associations with transparent junction tables, i.e. where junction tables only have two foreign keys to the associated entities. They simply didn't choose to support this far less common "skipping one-to-many-to-many" scenario in a similar manner.
And I can imagine why.
To start with, in a many-to-many association, the junction table is nothing but that: a junction, an association. However, in a chain of one-to-many (or many-to-one) associations it would be exceptional for any of the involved tables to be just an association. In your example...
Animal → AnimalCategory → Region
...AnimalCategory would only have a primary key (Id) and a foreign key (RegionId). That would be useless though: Animal might just as well have a RegionId itself. There's no reason to support a data model that doesn't make sense.
What you're after though, is a model in which the table in the middle does carry information (AnimalCategory.Name), but where you'd like to map it as a transparent junction table, because a particular class model doesn't need this information.
Your focus seems to be on reading data. But EF has to support all CRUD actions. The problem here would be: how to deal with inserts? Suppose Name is a required field. There would be no way to supply its value.
Another problem would be that a statement like...
region.Animals.Add(animal);
...could mean two things:
add an Animal and a new AnimalCategory, the latter referring to the Region.
Add an Animal referring to an existing AnimalCategory - without being able to choose which one.
EF wouldn't want to choose for some default behavior. You'd have to make the choice yourself, so you can't do without access to AnimalCategory.
I'm currenty working with Core-data for an iPhone project.
But I'm a bit confused about one element.
With Core Data currently you do not need to create the intermediate table when creating many to many relationships (its all handled behind the scenes by core data)
But in my case I actually need some attributes on my many to many relationship!
For example
I have a table called Students
and another table called Lessons
a Student can be in many lessons
and a lesson can have many students
Now a standard many to many relationship will not work for me as I actually need to define more details on the join, i.e. StartDate and LeaveDate.
In a standard sql model for example my join table would be something like
StudentLessons (Studentid, LessonId, StartDate, LeaveDate )
I would need these properties as when i'm querying for information I will need the details from the join to filter my results.
How can I create this in core data and also filter for results?
I've seen folks say that you would actually create the StudentLesson entity manually in core data.
Now if I did this would I just have the attributes (Startdate, LeaveDate) and then a one to many relationship from the Student and then the Lessons table?
Student - > StudentLessons
Lesson - > StudentLessons
I guess I'm a bit confused on how I would go about making sure that the relationships and the content of the relationships are setup correctly. (i.e If I add an Student object to the StudentLessons - how would I then assign/add the Lesson.)
Sorry this is my first time playing with Core Data.
Takes a bit o getting used to when coming from a full on sql background.
You are absolutely right. The correct way to do this is to create a new entity like StudentLessons. Let's call it Attendance. It should have the startDate and endDate, and two relationships.
The relationship to the student can be many-to-many, unless it is foreseeable that startDate and endDate are always different for each student. One Attendance with its dates can have many students in it. One student can have several Attendance duties.
Student <<---->> Attendance
Clearly, the relationship to Lesson should be one-to-many. One Lesson can have different Attendance configurations, with different dates. But each Attendance belongs only to one Lesson.
Lesson <---->> Attendance
To address your question, you can make the Attendance attribute of Lesson non-optional (and vice versa), this way it will ensure that each Lesson has at least one Attendance with appropriate dates, and each Attendance has exactly one Lesson.
I think your can remove the link between Student and Lesson. Just assign an Attendance rather than a lesson. If you want a Lesson assigned to a Student without dates, just allow Attendance to have NULL as those properties.
TheTiger,
Just because Core Data will create a join table for you, that doesn't mean you have to use it. Maintaining which student succeeds with which lesson is just the same except you will create the intermediate entity and then use the appropriate setters to build the relationships.
You will have to use more key paths and do relationship prefetching but those are straightforward to do.
Andrew
I'm willing to give MongoDB and CouchDB a serious try. So far I've worked a bit with Mongo, but I'm also intrigued by Couch's RESTful approach.
Having worked for years with relational DBs, I still don't get what is the best way to get some things done with non relational databases.
For example, if I have 1000 car shops and 1000 car types, I want to specify what kind of cars each shop sells. Each car has 100 features. Within a relational database i'd make a middle table to link each car shop with the car types it sells via IDs. What is the approach of No-sql? If every car shop sells 50 car types, it means replicating a huge amount of data, if I have to store within the car shop all the features of all the car types it sells!
Any help appreciated.
I can only speak to CouchDB.
The best way to stick your data in the db is to not normalize it at all beyond converting it to JSON. If that data is "cars" then stick all the data about every car in the database.
You then use map/reduce to create a normalized index of the data. So, if you want an index of every car, sorted first by shop, then by car-type you would emit each car with an index of [shop, car-type].
Map reduce seems a little scary at first, but you don't need to understand all the complicated stuff or even btrees, all you need to understand is how the key sorting works.
http://wiki.apache.org/couchdb/View_collation
With that alone you can create amazing normalized indexes over differing documents with the map reduce system in CouchDB.
In MongoDB an often used approach would be store a list of _ids of car types in each car shop. So no separate join table but still basically doing a client-side join.
Embedded documents become more relevant for cases that aren't many-to-many like this.
Coming from a HBase/BigTable point of view, typically you would completely denormalize your data, and use a "list" field, or multidimensional map column (see this link for a better description).
The word "column" is another loaded
word like "table" and "base" which
carries the emotional baggage of years
of RDBMS experience.
Instead, I find it easier to think
about this like a multidimensional map
- a map of maps if you will.
For your example for a many-to-many relationship, you can still create two tables, and use your multidimenstional map column to hold the relationship between the tables.
See the FAQ question 20 in the Hadoop/HBase FAQ:
Q:[Michael Dagaev] How would you
design an Hbase table for many-to-many
association between two entities, for
example Student and Course?
I would
define two tables: Student: student
id student data (name, address, ...)
courses (use course ids as column
qualifiers here) Course: course id
course data (name, syllabus, ...)
students (use student ids as column
qualifiers here) Does it make sense?
A[Jonathan Gray] : Your design does
make sense. As you said, you'd
probably have two column-families in
each of the Student and Course tables.
One for the data, another with a
column per student or course. For
example, a student row might look
like: Student : id/row/key = 1001
data:name = Student Name data:address
= 123 ABC St courses:2001 = (If you need more information about this
association, for example, if they are
on the waiting list) courses:2002 =
... This schema gives you fast access
to the queries, show all classes for a
student (student table, courses
family), or all students for a class
(courses table, students family).
In relational database, the concept is very clear: one table for cars with columns like "car_id, car_type, car_name, car_price", and another table for shops with columns "shop_id, car_id, shop_name, sale_count", the "car_id" links the two table together for data Ops. All the columns must well defined in creating the database.
No SQL database systems do not require you pre-define these columns and tables. You just construct your records in a certain format, say JSon, like:
"{car:[id:1, type:auto, name:ford], shop:[id:100, name:some_shop]}",
"{car:[id:2, type:auto, name:benz], shop:[id:105, name:my_shop]}",
.....
After your system is on-line providing service for your management, you may find there are some flaws in your design of db structure, you hope to add one column "employee" of "shop" for your future records. Then your new records coming is as:
"{car:[id:3, type:auto, name:RR], shop:[id:108, name:other_shop, employee:Bill]}",
No SQL systems allow you to do so, but relational database is impossible for this job.