错误：没有唯一约束匹配给定键的引用表

Question

I am getting an error when i create my tabels. The problem is that AssCode is not unique, so i can set it to unique, the combination of courseCode and AssCode is unique, thats why they are set as primary keys. I am using postgressql

here is the error:

ERROR: there is no unique constraint matching given keys for referenced table "assignments" SQL state: 42830

here is my code:

CREATE TABLE Teachers (
    BSN int primary key,
    Surname varchar(40) NOT NULL,
    Name varchar(40) NOT NULL   
);

CREATE TABLE Courses  (
    CourseCode varchar(10) primary key,
    Name varchar(20) NOT NULL
);

CREATE TABLE Assignments (
    CourseCode varchar(10) REFERENCES Courses ON DELETE CASCADE,
    AssCode varchar(10),
    primary key(CourseCode,AssCode),
    DependOn varchar(10),
    Year date,  
    week int
);

CREATE TABLE WorkOn ( 
    BSN int REFERENCES Teachers(BSN),
    CourseCode varchar(10)  REFERENCES Assignments(CourseCode),
    AssCode varchar(10) REFERENCES Assignments(AssCode),
    primary key (CourseCode,BSN,AssCode)
 );

Answer 1

I found the answer:

CREATE TABLE WorkOn ( 
    BSN int primary key REFERENCES Teachers(BSN),   
    CourseCode varchar(10),
    AssCode varchar(10),    
    foreign key (CourseCode, AssCode) references Assignments (CourseCode, AssCode)
);