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Asked: 2026-05-15T23:18:00+00:00

Could I declare a model with a key called :key , for instance? Is

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Could I declare a model with a key called :key, for instance? Is there any word I can’t use for a key?

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  1. Editorial Team

    The first question if very easy to answer yourself. Open irb and try:

    >> require 'mongo_mapper'
=> true
>> MongoMapper.database = 'test'
=> "test"
>> class Test
>>   include MongoMapper::Document
>>   key :key
>>   end
=> #<MongoMapper::Plugins::Keys::Key:0x101fc7a90 @default_value=nil, @type=nil, @name="key", @options={}>
>> t = Test.new(:key => 'value')
=> #<Test _id: BSON::ObjectID('4c4dcced7123374587000001'), key: "value">
>> t.save
=> true
>> Test.all
=> [#<Test _id: BSON::ObjectID('4c4dcced7123374587000001'), key: "value">]

    No errors? I guess key is a valid key!

    As far as I know, the only keys you shouldn’t use for your own data are _id and _type. You can use either, but they will change behavior. Using _id will make whatever you’re setting as that key the unique id for the object. Using _type will cause MongoMapper to try to instantiate an instance of whatever’s in your _test key when bringing the object back from the database.

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