addon/active-model-serializer.js
function(relationshipModelName, kind) {
    var key = decamelize(relationshipModelName);
    if (kind === "belongsTo") {
      return key + "_id";
    } else if (kind === "hasMany") {
      return singularize(key) + "_ids";
    } else {
      return key;
    }
  }
function(relationshipModelName, kind) {
    var key = decamelize(relationshipModelName);
    if (kind === "belongsTo") {
      return key + "_id";
    } else if (kind === "hasMany") {
      return singularize(key) + "_ids";
    } else {
      return key;
    }
  }
function ( relationshipModelName , kind ) { var key = decamelize ( relationshipModelName ) ; if ( kind === "belongsTo" ) { return key + "_id" ; } else if ( kind === "hasMany" ) { return singularize ( key ) + "_ids" ; } else { return key ; } }
Underscores relationship names and appends "_id" or "_ids" when serializing
relationship keys.

@method keyForRelationship
@param {String} relationshipModelName
@param {String} kind
@return String
Underscores relationship names and appends _id or _ids when serializing relationship keys .