Sequelize default foreign key - postgresql

Question

I have a model Route belongs to Region, when I define the associate, I thought you could just simply do the following:
"use strict";
module.exports = (sequelize, DataTypes) => {
const Route = sequelize.define("Route", {
title: {
type: DataTypes.STRING,
allowNull: false
}
});
Route.associate = models => {
Route.belongsTo(models.Region);
};
return Route;
};
With the route:
app.post("/api/regions/:regionId/routes", routesController.create);
Controller:
create(req, res) {
console.log(req.params.regionId);
return Route.create({
title: req.body.title,
regionId: req.params.regionId
})
.then(route => res.status(201).send(route))
.catch(err => res.status(400).send(err));
},
But when I post to that route, I keeps getting back the route with Null for regionId. Even though I can log the regionId with POST params.
I know the fix is to add foreignKey as following to explicit declare to use regionId as foreignKey:
Route.associate = models => {
Route.belongsTo(models.Region, {
foreignKey: "regionId"
});
};
But I thought it could just default regionId as foreignKey without declaring it. Am I missing something?