What Causes The INSERT statement conflicted with the FOREIGN KEY constraint?

Question

This code has worked for me before but i am not sure anymore what has been causing this error. My only guess is that when i try to create a Player, the Team Data is being se

Answer 1

You should try to complete entities like teams for your player entity. Especially foreign keys.

[HttpPost]
    public ActionResult Create(Player model)
    {
        if (ModelState.IsValid)
        {
            using (var db = new EfDb())
            {
                //Since you have the username cached, you can check the local EF cache for the object before hitting the db.
                //var userProfile = db.UserProfiles.Single(u => u.UserName == User.Identity.Name);
                var userProfile = db.UserProfiles.Local.SingleOrDefault(u => u.UserName == User.Identity.Name) 
                               ?? db.UserProfiles.SingleOrDefault(u => u.UserName == User.Identity.Name);
                if (userProfile != null)
                {
                    var player = new Player
                                     {
                                         UserProfile = userProfile,                                             
                                         ....
                                         ....                                              
                                     };  
                    player.TeamId = 5;                      
                    db.Players.Add(player);                          

                    db.SaveChanges();
                }
            }
        }
        PopulateTeamsDropDownList(model.TeamId);
        return View(model);
    }

And make sure the Dropdown from your view is Required and send correct value to the post method.

Answer 2

The reason behind this is when you are inserting a record on the child table which the value of the referencing column doesn't exist yet on the on the parent table.

Consider the following scenario:

// PARENT TABLE
CREATE TABLE TableA
(
   ID INT PRIMARY KEY
);

// CHILD TABLE
CREATE TABLE TableB
(
   ID INT PRIMARY KEY,
   TableA_ID INT,
   CONSTRAINT tb_FK FOREIGN KEY (TableA_ID) REFERENCES TableA(ID)
);


// RECORDS OF PARENT TABLE
INSERT INTO TableA (ID) VALUES (1);
INSERT INTO TableA (ID) VALUES (2);

// RECORDS OF CHILD TABLE
INSERT INTO TableB (ID, TableA_ID) VALUES (1,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (2,1);
INSERT INTO TableB (ID, TableA_ID) VALUES (3,2);

If you execute the statements above, it will not fail because none of them violates the referential integrity rule.

Try executing the following statement:

INSERT INTO TableB (ID, TableA_ID) VALUES (3,4);

It fails because 4 which is the value of TableA_ID to be inserted doesn't exist on Table1.ID. Foreign Keys preserved referential integrity between the records.