foreign-keys

问题 I have a Rails model which I use two has_one relations: requester and friend . When in the console I use: f = FriendRequest.all f[0].requester I get ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: users.requester_id: SELECT "users".* FROM "users" WHERE "users"."requester_id" = 4 LIMIT 1 . I don't really know how to specify a `has_one' relationship with a class name and a key which specifies the record. This is my model: class FriendRequest < ActiveRecord::Base has_one

问题 Suppose I have a field f in my model defined as follows as a foreign key: f = models.ForeignKey(AnotherModel) When Django syncs database, the filed created in db will be called f_id , automatically suffixed with '_id'. The problem is I want this field in db named exactly as what I defined in model, f in this case. How can I achieve that? 回答1: Well it turns out there's a keyword argument called db_column . If you want the field called 'f' in the database is just as simple as: f = models

问题 Suppose I have a field f in my model defined as follows as a foreign key: f = models.ForeignKey(AnotherModel) When Django syncs database, the filed created in db will be called f_id , automatically suffixed with '_id'. The problem is I want this field in db named exactly as what I defined in model, f in this case. How can I achieve that? 回答1: Well it turns out there's a keyword argument called db_column . If you want the field called 'f' in the database is just as simple as: f = models

问题 I have a Django project that has multiple django "apps". One of them has models to represent data coming from an external source (I do not control this data). I want my other apps to be able to have references to this "external app" but I want to avoid all the fuzz of the database integrity checks. I don't want the db to have any constraints on these "soft foreign keys". Do you know how I can code a custom field that will emulate a real Django ForeignKey without creating a hard constraint on

问题 I have table 1 with a primary key user_id and table 2 where user_id is a foreign key. Only 1 record per user_id can exist in table 2, and no record can exist without it. QUESTION: Can user_id in table 2 be both foreign and primary key at the same time, and if yes, is it a good idea, what are pros/cons? 回答1: Yes, you can do this (and you should, from a database design point of view). However, consider what it means if user_id is the primary key on table 2. You are in effect saying that each

问题 I am working with a large PostgreSQL database, and I am trying to tune it to get more performance. Our queries and updates seem to be doing a lot of lookups using foreign keys. What I would like is a relatively simple way to add Indexes to all of our foreign keys without having to go through every table (~140) and doing it manually. In researching this, I've come to find that there is no way to have Postgres do this for you automatically (like MySQL does), but I would be happy to hear

问题 I was wondering can some give me an explanation on how to assign primary and foreign keys in pgAdmin? I can't find any information online. For example...I've got a Student table with all their details (address, d.o.b. and etc.). I'm going to add a student_number to the table and make it a primary key. I just want to know how do I do that using pgAdmin? And if you may be kind to explain give me further information on using Primary Keys in postgreSQL (and pgAdmin). The same case with the

问题 I'm using PostgreSQL and I'm trying to list all the tables that have a particular column from a table as a foreign-key/reference. Can this be done? I'm sure this information is stored somewhere in information_schema but I have no idea how to start querying it. 回答1: select R.TABLE_NAME from INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE u inner join INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS FK on U.CONSTRAINT_CATALOG = FK.UNIQUE_CONSTRAINT_CATALOG and U.CONSTRAINT_SCHEMA = FK.UNIQUE_CONSTRAINT

问题 There is an username/password verification step first then the database has following structure ^ is primary key * uses foreign key 1.StudentDetails table =========================================================================== ID^| Username | Password | Email | Address * | Website |Comments ====+============+==========+=============+===========+=========+========== 1 | xxxxxxxxxx | xxxxxxx | xx@xxx.xxx | 1 | http:// | text 2.Submissions table ==============================================

问题 I have SQLite 3 with the following setup (simplified): create table Location(LocationId integer not null, LocationCode text not null, primary key(LocationId), unique(LocationCode)); The table above is being referenced by Department: create table Department(DepartmentId integer not null, LocationId integer not null, DepartmentCode text not null, primary key(LocationId, DepartmentCode), foreign key(LocationId) references Location(LocationId)); The table above is being referenced by Child: