How to fix InvalidForeignKey error?

Question

Python

Explain how to fix this error with FK.
The code:

cur.execute('''CREATE TABLE Vm
    (id_vm INT PRIMARY KEY NOT NULL,
    uuid_vm CHAR(100) NOT NULL,
    name_vm CHAR(100) NOT NULL,
    bg_date DATE NOT NULL,
    end_date DATE DEFAULT('2999-05-12'))
    ''')
cur.execute('''CREATE TABLE Vm_parm
    (id_parm INT PRIMARY KEY NOT NULL,
    title CHAR(30) NOT NULL,
    type CHAR(10) NOT NULL)
    ''')
cur.execute('''CREATE TABLE Vm_stat
    (id_stat INT PRIMARY KEY NOT NULL,
    uuid_vm CHAR(100) NOT NULL,
    name_vm CHAR(100) NOT NULL,
    bg_date DATE NOT NULL,
    end_date DATE NOT NULL,
    vm_parm CHAR(30) NOT NULL,
    value_parm INT NOT NULL)
    ''')
cur.execute("ALTER TABLE Vm_stat ADD CONSTRAINT fk_Vm_stat_uuid_vm_name_vm_bg_date_end_date FOREIGN KEY(uuid_vm, name_vm, bg_date, end_date)"
            "REFERENCES Vm (uuid_vm, name_vm, bg_date, end_date)")
cur.execute("ALTER TABLE Vm_stat ADD CONSTRAINT fk_Vm_stat_vm_parm FOREIGN KEY(title)"
            "REFERENCES Vm_parm(title)")

Gives me this error:

line 34, in <module>
    cur.execute("ALTER TABLE Vm_stat ADD CONSTRAINT fk_Vm_stat_uuid_vm_name_vm_bg_date_end_date FOREIGN KEY (uuid_vm, name_vm, bg_date, end_date)"
psycopg2.errors.InvalidForeignKey: there is no unique constraint matching given keys for referenced table "vm"

Answer 1

Well, he writes in English - there is no unique constraint on the group of fields that you refer to (uuid_vm, name_vm, bg_date, end_date).
In general, they piled up a strange scheme: if there is a surrogate key id_vm, why keep a bunch of fields in both tables, and even make a foreign key on them?