только путь к баз замени на свой

package main

import (
    "github.com/jinzhu/gorm"
    _ "github.com/lib/pq"

    //"fmt"
    "log"
)

// has_many Ys
type X struct {
    gorm.Model
}

// belongs_to X
type Y struct {
    gorm.Model
    XID uint
}

func main() {
    db, err := gorm.Open("postgres", "postgres://user:psw@localhost/db")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()
    db.LogMode(true)
    db.AutoMigrate(X{}, Y{})
    db.Model(&Y{}).AddForeignKey("x_id", "xes (id)", "CASCADE", "CASCADE")
}

/*

=====================       Вот такой лог ======================


(/home/anoobis/pro/gorm/main.go:29)
[2016-03-23 00:13:06]  [157.16ms]  CREATE TABLE "xes" ("id" serial,"created_at" timestamp with time zone,"updated_at" timestamp with time zone,"deleted_at" timestamp with time zone , PRIMARY KEY ("id"))

(/home/anoobis/pro/gorm/main.go:29)
[2016-03-23 00:13:06]  [88.58ms]  CREATE INDEX idx_xes_deleted_at ON "xes"("deleted_at")

(/home/anoobis/pro/gorm/main.go:29)
[2016-03-23 00:13:07]  [111.14ms]  CREATE TABLE "ys" ("id" serial,"created_at" timestamp with time zone,"updated_at" timestamp with time zone,"deleted_at" timestamp with time zone,"x_id" integer , PRIMARY KEY ("id"))

(/home/anoobis/pro/gorm/main.go:29)
[2016-03-23 00:13:07]  [92.48ms]  CREATE INDEX idx_ys_deleted_at ON "ys"("deleted_at")

(/home/anoobis/pro/gorm/main.go:30)
[2016-03-23 00:13:07]  [142.14ms]  ALTER TABLE "ys" ADD CONSTRAINT "ys_x_id_xes_id_foreign" FOREIGN KEY ("x_id") REFERENCES xes (id) ON DELETE CASCADE ON UPDATE CASCADE;


===================== собственно ==========================

ALTER TABLE "ys" ADD CONSTRAINT "ys_x_id_xes_id_foreign" FOREIGN KEY ("x_id") REFERENCES xes (id) ON DELETE CASCADE ON UPDATE CASCADE;

И есть добавление ограничения внешнего ключа к x_id (таблици ys) - так чтоб он полюбому соответствовал какому-либо
существующему id в таблице xes.


=================== в psql =========================

 \d xes
                                    Table "public.xes"
   Column   |           Type           |                    Modifiers
------------+--------------------------+--------------------------------------------------
 id         | integer                  | not null default nextval('xes_id_seq'::regclass)
 created_at | timestamp with time zone |
 updated_at | timestamp with time zone |
 deleted_at | timestamp with time zone |
Indexes:
    "xes_pkey" PRIMARY KEY, btree (id)
    "idx_xes_deleted_at" btree (deleted_at)
Referenced by:
    TABLE "ys" CONSTRAINT "ys_x_id_xes_id_foreign" FOREIGN KEY (x_id) REFERENCES xes(id) ON UPDATE CASCADE ON DELETE CASCADE

====================== и ==================

\d ys
                                    Table "public.ys"
   Column   |           Type           |                    Modifiers
------------+--------------------------+-------------------------------------------------
 id         | integer                  | not null default nextval('ys_id_seq'::regclass)
 created_at | timestamp with time zone |
 updated_at | timestamp with time zone |
 deleted_at | timestamp with time zone |
 x_id       | integer                  |
Indexes:
    "ys_pkey" PRIMARY KEY, btree (id)
    "idx_ys_deleted_at" btree (deleted_at)
Foreign-key constraints:
    "ys_x_id_xes_id_foreign" FOREIGN KEY (x_id) REFERENCES xes(id) ON UPDATE CASCADE ON DELETE CASCADE


=================================== проверка ограничения ======================

INSERT INTO ys (created_at, x_id) VALUES (now(), 90);
ERROR:  insert or update on table "ys" violates foreign key constraint "ys_x_id_xes_id_foreign"
DETAIL:  Key (x_id)=(90) is not present in table "xes".

*/