Zliczanie powiązanych wpisów z Sequelize

Użyj findAll() z include() i sequelize.fn() dla COUNT :

Location.findAll({
    attributes: { 
        include: [[Sequelize.fn("COUNT", Sequelize.col("sensors.id")), "sensorCount"]] 
    },
    include: [{
        model: Sensor, attributes: []
    }]
});

Lub może być konieczne dodanie group a także:

Location.findAll({
    attributes: { 
        include: [[Sequelize.fn("COUNT", Sequelize.col("sensors.id")), "sensorCount"]] 
    },
    include: [{
        model: Sensor, attributes: []
    }],
    group: ['Location.id']
})