Postgresql 外键作为主键不唯一？

Question

So i have a table, that takes a nickname as foreign key and a id to form a primary key:

CREATE TABLE Character(
    Nickname TEXT,
    CONSTRAINT person_pk PRIMARY KEY(Nickname)
);
CREATE TABLE POSTING(
    PostingID BIGINT UNIQUE, 
    Nickname TEXT,
CONSTRAINT posting_fk FOREIGN KEY(Nickname) REFERENCES Person(Nickname),
    CONSTRAINT postings_pk PRIMARY KEY(PostingID, Nickname)
);

So far, so good. However, whenever i try to get now the primary key from posting, it tells me that: " there is no unique constraint matching given keys for referenced table "information"".

CREATE TABLE INFORMATION(
    InformationID BIGINT,
    PostingID BIGINT, 
    CONSTRAINT informations_fk FOREIGN KEY(PostingID) REFERENCES POSTING(PostingID),
    CONSTRAINT informations_pk PRIMARY KEY(InformationID, PostingID)
);


CREATE TABLE DATA(
    InformationID BIGINT,
    Link TEXT NOT NULL,
    CONSTRAINT data_fk FOREIGN KEY(InformationID) REFERENCES INFORMATION(InformationID),
    CONSTRAINT datas_pk PRIMARY KEY(InformationID)
);

However, if i change in the table Information the InformationID to unique, the error disappears. Is that the correct way of doing things?

CREATE TABLE INFORMATION(
    InformationID BIGINT UNIQUE,
    PostingID BIGINT,
    CONSTRAINT informations_fk FOREIGN KEY(PostingID) REFERENCES POSTING(PostingID),
    CONSTRAINT informations_pk PRIMARY KEY(InformationID, PostingID)
);

Thanks in advance everyone!

Answer 1

Either the primary key for information should be only informationid , or data must include postingid and the foreign key to information is defined on both columns.

Hard to say without knowing your data, but I suspect the former solution to be the correct one.