Q:

Problemas para obter token oauth2

Estou tendo problemas para obter o token do oauth2
Estou tentando pelo postman pegar o token. 
Tenho cadastrado no mongo o usuario que estou tentando obter um token

{
    "statusCode": 400,
    "status": 400,
    "code": 400,
    "message": "Invalid client: cannot retrieve client credentials",
    "name": "invalid_client"
}

OAuth2Model.js
/**
 * Module dependencies.
 */
const debug = require('debug')('OAuth2Model');
var mongoose = require('mongoose');
var Schema = mongoose.Schema;

/**
 * Schema definitions.
 */

mongoose.model('OAuthTokens', new Schema({
    accessToken: { type: String },
    accessTokenExpiresAt: { type: Date },
    client : { type: Object },  // `client` and `user` are required in multiple places, for example `getAccessToken()`
    clientId: { type: String },
    refreshToken: { type: String },
    refreshTokenExpiresAt: { type: Date },
    user : { type: Object },
    userId: { type: String },
  }));

  mongoose.model('OAuthClients', new Schema({
    clientId: { type: String },
    clientSecret: { type: String },
    redirectUris: { type: Array }
  }));

  mongoose.model('OAuthUsers', new Schema({
    email: { type: String, default: '' },
    firstname: { type: String },
    lastname: { type: String },
    password: { type: String },
    username: { type: String }
  }));

  var OAuthTokensModel = mongoose.model('OAuthTokens');
  var OAuthClientsModel = mongoose.model('OAuthClients');
  var OAuthUsersModel = mongoose.model('OAuthUsers');

  /**
 * Get access token.
 */

module.exports.getAccessToken = async function(bearerToken) {

    const tokenObj = await OAuthTokensModel.findOne({ accessToken: bearerToken }).lean();
    return tokenObj;
  };

  /**
   * Get client.
   */

  module.exports.getClient = async function(clientId, clientSecret) {

      const clientDetails =  await OAuthClientsModel.findOne({ clientId: clientId, clientSecret: clientSecret }).lean();
      return clientDetails;
  };

  /**
   * Get refresh token.
   */

  module.exports.getRefreshToken = async function(refreshToken) {
    return await OAuthTokensModel.findOne({ refreshToken: refreshToken }).lean();
  };

  /**
   * Get user.
   */

  module.exports.getUser = async function(username, password) {
    return await OAuthUsersModel.findOne({ username: username, password: password }).lean();
  };

  /**
   * Save token.
   */

  module.exports.saveToken = function(token, client, user) {
    var accessToken = new OAuthTokensModel({
      accessToken: token.accessToken,
      accessTokenExpiresAt: new Date(token.accessTokenExpiresAt),
      client : client,
      clientId: client.clientId,
      refreshToken: token.refreshToken,
      refreshTokenExpiresAt: new Date(token.refreshTokenExpiresAt),
      user : user,
      userId: user._id,
    });
    console.log("accesss token",accessToken)
    return new Promise( function(resolve,reject){
      accessToken.save(function(err,data){
        if( err ) reject( err );
        else resolve( data );
      }) ;
    }).then(function(saveResult){
      saveResult = saveResult && typeof saveResult == 'object' ? saveResult.toJSON() : saveResult;

      var data = new Object();
      for( var prop in saveResult ) data[prop] = saveResult[prop];

      data.client = data.clientId;
      data.user = data.userId;

      return data;
    });
  };

OAuth2Service
const debug = require('debug')('OAuth2Service');
const OAuth2Model = require('../model/OAuth2Model.js');
const OAuth2Server = require('oauth2-server'),     //Represents an OAuth2 server instance.
    Request = OAuth2Server.Request,
    Response = OAuth2Server.Response;
var instance;
/**
 * Instantiates OAuth2Server using the supplied model.
 */
var oAuth2 = new OAuth2Server({
    model:OAuth2Model,
    accessTokenLifetime: 86500,
    allowBearerTokensInQueryString: true
});
/**
 * Creating constructor
 */
function OAuth2Service(){
}
/**
 * Define the shared properties and methods using the prototype
 */

/**
 * Obtaine OAuth token with Basic Authentication
 */
OAuth2Service.prototype.obtainToken = function(req, res) {
    var request = new Request(req);
    var response = new Response(res);
    return oAuth2.token(request, response)
        .then(function(token) {
            debug("obtainToken: token %s obtained successfully",token);
            res.json(token);
        }).catch(function(err) {

            res.status(err.code || 500).json(err);
        });
}
/**
 * Authenticates a request.
 */
OAuth2Service.prototype.authenticateRequest = function(req, res, next) {
    var request = new Request(req);
    var response = new Response(res);
    return oAuth2.authenticate(request, response)
        .then(function(token) {
            debug("the request was successfully authenticated")
            next();
        }).catch(function(err) {

            res.status(err.code || 500).json(err);
        });
}
/**
 * Export an Instance
 */
module.exports = {
    getInstance: function () {
        if (!instance) {
            instance = new OAuth2Service();
        }

        return instance;
    }
};

A:

Problema solucionado, a collection que eu havia criado estava com letras maiúsculas. E o mongoose não conseguia encontrar. Alterei para tudo minusculo e Funcionou;

