Next.js API Routing

Try doing like so... (this post is a bit long, but if you follow the logic and import order, it should work):

Example project structure:

├── .next
|
├── database
|   └── index.js
|
├── models
|   ├── all.js
|   ├── index.js
|   └── teams.js
|
├── src
|   └── pages
|       ├── api
|       |   └── teams
|       |       └── names.js
|       └── index.js
|
└── next.config.json

database/index.js -- this will establish a connection to a mongo database using mongoose

const bluebird = require("bluebird");
const mongoose = require("mongoose");

// I use process.env.DATABASE as a flexible database name
// this can be set in an ".env" file or in your package.json scripts...
// like "dev" : "DATABASE=example-dev next dev"
const { DATABASE } = process.env;

const options = {
    useNewUrlParser: true, // avoids DeprecationWarning: current URL string parser is deprecated
    useCreateIndex: true, // avoids DeprecationWarning: collection.ensureIndex is deprecated.
    useFindAndModify: false, // avoids DeprecationWarning: collection.findAndModify is deprecated.
    useUnifiedTopology: true, // avoids DeprecationWarning: current Server Discovery and Monitoring engine is deprecated
};

// connects to a mongodb database
mongoose.connect(`mongodb://localhost/${DATABASE}`, options); 

// uses bluebird for mongoose promises
mongoose.Promise = bluebird; 

models/all.js -- this will require all the models to be registered to the database connection

require("./team");
require("./user");
...etc

models/index.js -- this will export the mongoose model instances (you don't need this file, as you can just import mongoose and retrieve the model(Team) from within a file, but I like to reduce repetitive imports)

const { model } = require("mongoose");

module.exports = {
    Team: model("Team"),
    User: model("User"),
    ...etc
};

models/team.js -- this will establish the schema as a mongoose model

const { Schema, model } = require("mongoose");

const teamSchema = new Schema({
    league: { type: String, required: true },
    team: { type: String, unique: true },
    name: { type: String, unique: true, lowercase: true },
});

module.exports = model("Team", teamSchema);

pages/api/teams/all.js -- import the model instance from within an API route...

import { Team } from "../../../models"; (index.js)
// alternatively: 
// import { model } from "mongoose";
// cost Team = model("Team");

/**
 * Retrieves all teams names.
 *
 * @function getAllTeamNames
 * @returns {object} - names
 * @throws {string}
 */
const getAllTeamNames = async (_, res) => {
    // aggregates all team "names" into a single array, like so:
    // [{ names: ["team1,"team2","team3","team4", ...etc] }]

    const teams = await Team.aggregate([
        { $group: { _id: null, names: { $addToSet: "$team" } } },
        { $unwind: "$names" },
        { $sort: { names: 1 } },
        { $group: { _id: null, names: { $push: "$names" } } },
        { $project: { _id: 0, names: 1 } },
    ]); 

    // returns "names" array (["team1,"team2","team3","team4", ...etc])
    res.status(200).json({ names: teams[0].names });
};

export default getAllTeamNames;

next.config.js -- this will establish a connection pool and register models before next loads

require("./database"); // establishes the mongo connection
require("./models/all"); // registers the models to the connection

...etc