moved require's out of functions and declared it at the top of the file

lib/associations/mixin.js

-var Utils = require("./../utils")
+var Utils     = require("./../utils")
+  , HasOne    = require('./has-one')
+  , HasMany   = require("./has-many")
+  , BelongsTo = require("./belongs-to")
 
 /* Defines Mixin for all models. */
 var Mixin = module.exports = function(){}
 
 Mixin.hasOne = function(associatedModel, options) {
   // the id is in the foreign table
-  var HasOne = require('./has-one')
   var association = new HasOne(this, associatedModel, Utils._.extend((options||{}), this.options))
   this.associations[association.associationAccessor] = association.injectAttributes()
   return this
 }
 Mixin.belongsTo = function(associatedModel, options) {
   // the id is in this table
-  var BelongsTo = require("./belongs-to")
   var association = new BelongsTo(this, associatedModel, Utils._.extend((options||{}), this.options))
   this.associations[association.associationAccessor] = association.injectAttributes()
   return this
 }
 Mixin.hasMany = function(associatedModel, options) {
   // the id is in the foreign table or in a connecting table
-  var HasMany = require("./has-many")
   var association = new HasMany(this, associatedModel, Utils._.extend((options||{}), this.options))
   this.associations[association.associationAccessor] = association.injectAttributes()
   return this