error

TypeError: Cannot read property 'map' of undefined
    at json-serializer.js:584
    at ext.js:497
    at Map.forEach.cb (ember.debug.js:16815)
    at OrderedSet.forEach (ember.debug.js:16614)
    at Map.forEach (ember.debug.js:16819)
    at Function.ember$data$lib$system$model$$default.reopenClass.eachRelationship (ext.js:496)
    at ember$data$lib$system$serializer$$default.extend.extractRelationships (json-serializer.js:575)
    at ember$data$lib$system$serializer$$default.extend.normalize (json-serializer.js:487)
    at superFunction [as _super] (ember.debug.js:17013)
    at ember$data$lib$serializers$json$serializer$$default.extend.normalize (rest-serializer.js:115)
    
    
    
    } else if (relationshipMeta.kind === 'hasMany') {
          data = relationshipHash.map(function (item) {
            return _this4.extractRelationship(relationshipMeta.type, item);
          });
        }
        
        relationshipHash == undefined

model.js

export default DS.Model.extend({
   	customer_group: DS.belongsTo('customer-group'),
   	position: DS.attr('number',{defaultValue: 0})
});

payload.json

{
    customer_groups: [{
        customer_ids: [],
        branch_opening_time_ids: [],
        id: 30,
        name: "XXX",
        customer_group_id: 37,
        change_date: "2015-03-19T00:00:00.000Z",
        conflict: null,
        updated: null,
        created_at: "2015-03-20T09:11:32.000Z",
        updated_at: "2015-03-20T09:11:32.000Z",
        note: null
    }, {
        .
        .
        .
        .
    }],
    dashboard_customer_groups: [{
        id: 96,
        position: null,
        customer_group_id: 30
    }, {
      .
      .
      .
    }
    }]
}

serializer.js

export default DS.RESTSerializer.extend({
    primaryKey: 'id',

    normalize: function (type, hash, prop) {

        hash.customer_group = hash.customer_group_id;
        delete hash.customer_group_id;

        return this._super(type, hash, prop);
    },

    serializeBelongsTo: function (record, json, relationship) {
        this._super(record, json, relationship);

        switch (relationship.key) {
        case 'customer_group':
            json.customer_group_id = json.customer_group;
            delete json.customer_group;
            break;
        }
    },

    serializeIntoHash: function(hash, type, record, options) {
        hash['dashboard_customer_group'] = this.serialize(record, options);
    },
});