CONSTRAINT UserGroups_userID_fkey FOREIGN KEY (userID) REFERENCES Users(id) ON UPDATE CASCADE ON DELETE NO ACTION

CREATE TRIGGER genfkey9_insert_referencing BEFORE INSERT ON "UserGroups" WHEN 
    new."userID" IS NOT NULL AND NOT EXISTS (SELECT  1  FROM "Users" WHERE new."userID" == "id")
BEGIN
  SELECT RAISE(ABORT, 'constraint failed');
END;
...

'insert violates constraint "UserGroups_userID_fkey": key is not present in table "Users"

'insert violates constraint "UserGroups_userID_fkey": key [userID = ' || new."userID" || '] is not present in table "Users"

CREATE TRIGGER genfkey9_insert_referencing BEFORE INSERT ON "UserGroups" WHEN 
    new."userID" IS NOT NULL AND NOT EXISTS (SELECT  1  FROM "Users" WHERE new."userID" == "id")
BEGIN
  SELECT RAISE(ABORT, 'insert violates constraint "genfkey9_insert_referencing": key [userID =' || new."userID" || '] is not present in table "Users"');
END;

Error: near "||": syntax error

CREATE TRIGGER genfkey9_insert_referencing BEFORE INSERT ON "UserGroups" WHEN 
    new."userID" IS NOT NULL AND NOT EXISTS (SELECT  1  FROM "Users" WHERE new."userID" == "id")
BEGIN
  SELECT RAISE(ABORT, new.userID);
END;

Error: near ".": syntax error

Error: userID