Foreign key to assotiation table - sql

Question

Is it OK to use a reference to a many-to-many association table or explicit FK's are better?
Let's assume we have some many-to-many model like this:
CREATE TABLE Project (
id int NOT NULL,
....
PRIMARY KEY(id)
);
CREATE TABLE Programmer (
id int NOT NULL,
....
PRIMARY KEY(id)
);
CREATE TABLE Project_Programmer_Association(
id int NOT NULL,
project_id int,
programmer_id int,
.... (probably some other data)
FOREIGN KEY (project_id) REFERENCES Project (id),
FOREIGN KEY (programmer_id) REFERENCES Programmer(id)
);
CREATE TABLE Task (
id int NOT NULL,
project_programmer_id int,
....
PRIMARY KEY(id),
FOREIGN KEY (project_programmer_id) REFERENCES Project_Programmer_Association(id)
);
Within Project and Programmer table there are some data, required for the Task, so I need references to both tables. My question is: Should I use this reference in Task
CREATE TABLE Task (
id int NOT NULL,
project_programmer_id int,
....
PRIMARY KEY(id),
FOREIGN KEY (project_programmer_id) REFERENCES Project_Programmer_Association(id)
);
or two explicit keys will be better (exept of additional join absence)
CREATE TABLE Task (
id int NOT NULL,
project_id int,
programmer_id int,
....
PRIMARY KEY(id),
FOREIGN KEY (project_id) REFERENCES Project (id),
FOREIGN KEY (programmer_id) REFERENCES Programmer(id)
);

Answer 1

I would prefer the following schema:
CREATE TABLE Project (
id int NOT NULL,
....
PRIMARY KEY(id)
);
CREATE TABLE Programmer (
id int NOT NULL,
....
PRIMARY KEY(id)
);
CREATE TABLE Project_Programmer_Association(
project_id int,
programmer_id int,
PRIMARY KEY(project_id, programmer_id),
FOREIGN KEY (project_id) REFERENCES Project,
FOREIGN KEY (programmer_id) REFERENCES Programmer
);
CREATE TABLE Task (
id int NOT NULL,
project_id int,
programmer_id int,
PRIMARY KEY(id),
FOREIGN KEY (project_id, programmer_id) REFERENCES Project_Programmer_Association
);
since it has the following advantages: no unnecessary key for Project_Programmer_Association, referential integrity guaranteed for Task; the possibility of joining directly Tasks with Project_Programmer_Association, Project and Programmer. Note that in the case for which there is only a task for each couple programmer-project (and only in that case) than you can remove also the id of Task and make the primary key the other two attributes.

Answer 2

The right question isn't "which is better", but "which represents your data".
If Project_Programmer_Association doesn't hold any data of its own, it may be redundant to keep it, and you could get away with foreign keys to the project and the programmer.
If it could hold data, and more specifically, if a project and a programmer can have several associations (e.g., Bob is the lead developer in project X but just a consultant in project Y), you most definitely need a foreign key to the association itself in order to properly represent your data model.