main.belongsToMany(left,{through: m_l});
left.belongsToMany(main,{through: m_l});

data.belongsToMany(left,{through: l_d});
left.belongsToMany(data,{through: l_d});

l_d.belongsToMany(main,{through: ld_m});
main.belongsToMany(l_d,{through: ld_m});
-------------------------------
router.post('/create-link-ldm',(request,response)=>{
    models.db.main.findOne({
        where:{id: request.body.main}
    })
        .then(main=>{
            models.db.l_d.findOne({
                where: {id: request.body.ld}
            })
                .then(ld=>{
                    main.addLd(ld);
                    response.json({
                        add: true
                    });
                })
                .catch(err=>console.log(err));
        })
        .catch(err=>console.log(err));
});