I would like to implement a bidirectional friendship relationship using SQLAlchemy.
User A <------> User B <------> User C User D
Can someone give me an example / code snippet of this?
You need a many to many JOIN table, because each individual can have many friends. The table will have a composite primary key consisting of two foreign keys that refer back to the individual table.
Related
I have two tables
student(sid,sname,sage,sgender,semail)
Course(cid,cname,credit)
where one student can enroll for only 1 course and in one course there can be many students.
so should I use the foreign key in the course table or creating a new table for enrollment.
Is this approach depends on the situation or others?
Based on your current scenario, you should create foreign key in your students table similar like s_cid.
But rather I suggest create separate table for enrollment, thinking of possible future requirement change where student can attend multiple courses. So you would have enrollment table with id, sid, cid columns.
I can't imagine the relationships it takes to find the best way the two following tables:
I have a table called "Poeples". It contains "ID, name and sex".
And I have an another table called "Pairs". It contains "ID, name, father_id, mother_id".
A person can belong to several pairs. And a pair can only have one parent_id and mother_id
I don't know if I need to make a foreign key for mother_id and father_id. I would like mother_id to be a poeple and father_id also a poeple (from the poeples table).
Do you have an idea of how?
I plan to use the belongsTo and hasMany relationship, but since there are two foreign keys (I do not know if that's what I have to do), I do not know how to do it.
Thank you very much
You can't make one relation for two fields.
What you need to do is to define two relations in Pair model, one would be PeopleByMother and another PeopleByFather (maybe find better naming).
Therefore a Pair model has two hasOne relations, and People has one belongsTo relation that aims for Pair model
I'm creating a website and users can add friends. I want them to be able to see their personal friends on a page.
For exemple:
John add user Tim and Bill.
When John goes on his friends list page, I want him to be able to see that he has Tim and Bill. How do I do that? Is that even possible? Do I need more than one table? If so, does every user has to have his own friendsList table?
Yes this is possible, you do this by querying the information from the database, the answer for if you need multiple tables etc all depends on your current table structure but at the very least you need to have some way of referencing that a Person 'John' has friends, wether thats just a 'friendID' in the same 'Person' table, or another means of doing so. then it is just a matter of querying the data correctly to return what you want and bind to the websites fields :D
One way of defining the structure is the following:
Person
PersonId
Name
<other person fields>
Relationship
RelationshipId
Name --> allow to define multiple relation types like Friendship, Follows etc.
Relationship
RelationshipId
Person1Id --> FK to Person
Person2Id --> FK to Person
RelationshipTypeId --> FK Relationship
Basically, you use an n:n between Persons (anyone can have any number of friends) and also allow for other types of relationships.
Assuming you already have a table of users, one approach would be to create a "friends" table which relates users to other users.
CREATE TABLE friends (
`user_id` INT NOT NULL,
`friend_id` INT NOT NULL
);
Both user_id and friend_id would have foreign key constraints on your existing users table (so that you guarantee an id must exist in your user table in order for it to exist in the friends table as either a user_id or friend_id).
You can then link your user table on users.id = friends.friend_id to get the friend's info.
Here is a SQL Fiddle Demo showing how this works.
You should consider using an ON DELETE CASCADE constraint on the friends table, so that if a user is deleted from the user table, the associated records in the friends table are also deleted.
I am using DBDesigner 4 for designing my database relations.
I have a users table and a recipes table. One user can own many recipes but one recipe cannot be owned by many users. This relationship is shown by the user_recipes relation in the picture. (A one-to-many relationship from users to recipes).
However, recipes can be liked by users. Many users can like many recipes. This is a many-to-many relationship between users and recipes and the pivot table for this is users_like_recipes.
But when I create this pivot table, I only need the users_id and recipes_id column. The recipes_users_id column is getting added on its own and I am not able to remove it. It says the third column has come from another Relation which is defined in the model. I guess its the user_recipes relation.
When I remove the user_recipes relation, I get the pivot table like I want to.
But I need the user_recipes relation too!
Please. Any help would be appreciated.
I would suggest removing user_id as a primary key from from the recipes table. Currently the combination if id and user_id provides identification for your recipes table. In this situation multiple user_id's can create the same recipe id because the combination has to be unique. user_id can just be a normal column in your table. If you REALLY want to, you can make an alternate key on (id, user_id) but you do not need it because the id is unique.
I'm currently designing a database structure for our team's project. I have this very question in mind currently: Is it possible to have a foreign key act as a primary key on another table?
Here are some of the tables of our system's database design:
user_accounts
students
guidance_counselors
What I wanted to happen is that the user_accounts table should contain the IDs (supposedly the login credential to the system) and passwords of both the student users and guidance counselor users. In short, the primary keys of both the students and guidance_counselors table are also the foreign key from the user_accounts table. But I am not sure if it is allowed.
Another question is: a student_rec table also exists, which requires a student_number (which is the user_id in the user_accounts table) and a guidance_counsellor_id (which is also the user_id in the user_accounts) for each of its record. If both the IDs of a student and guidance counselor come from the user_accounts table, how would I design the student_rec table? And for future reference, how do I manually write it as an SQL code?
This has been bugging me and I can't find any specific or sure answer to my questions.
Of course. This is a common technique known as supertyping tables. As in your example, the idea is that one table contains a superset of entities and has common attributes describing a general entity, and other tables contain subsets of those entities with specific attributes. It's not unlike a simple class hierarchy in object-oriented design.
For your second question, one table can have two columns which are separately foreign keys to the same other table. When the database builds the query, it joins that other table twice. To illustrate in a SQL query (not sure about MySQL syntax, I haven't used it in a long time, so this is MS SQL syntax specifically), you would give that table two distinct aliases when selecting data. Something like this:
SELECT
student_accounts.name AS student_name,
counselor_accounts.name AS counselor_name
FROM
student_rec
INNER JOIN user_accounts AS student_accounts
ON student_rec.student_number = student_accounts.user_id
INNER JOIN user_accounts AS counselor_accounts
ON student_rec.guidance_counselor_id = counselor_accounts.user_id
This essentially takes the student_rec table and combines it with the user_accounts table twice, once on each column, and assigns two different aliases when combining them so as to tell them apart.
Yes, there should be no problem. Foreign keys and primary keys are orthogonal to each other, it's fine for a column or a set of columns to be both the primary key for that table (which requires them to be unique) and also to be associated with a primary key / unique constraint in another table.