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    <p>Revised: I revised the answer to use before_create and building, not creating, the associated models. The ActiveRecord machinery then takes care of saving the associated models once the parent is saved. </p> <p>I even tested this code!</p> <pre><code># in your Room model... has_many :doors before_create :build_main_door private def build_main_door # Build main door instance. Will use default params. One param (:main) is # set explicitly. The foreign key to the owning Room model is set doors.build(:main =&gt; true) true # Always return true in callbacks as the normal 'continue' state end ####### has_one case: # in your Room model... has_one :door before_create :build_main_door private def build_main_door # Build main door instance. Will use default params. One param (:main) is # set explicitly. The foreign key to the owning Room model is set build_door(:main =&gt; true) true # Always return true in callbacks as the normal 'continue' state end </code></pre> <p>Added...</p> <p>The build method is added by the owning model's machinery by the has_many statement. Since the example uses has_many :doors (model name Door), the build call is doors.build</p> <p>See the <a href="http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#M001316" rel="noreferrer">docs for has_many</a> and <a href="http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#M001317" rel="noreferrer">has_one</a> to see all of the additional methods that are added.</p> <pre><code># If the owning model has has_many :user_infos # note: use plural form # then use user_infos.build(...) # note: use plural form # If the owning model has has_one :user_info # note: use singular form # then use build_user_info(...) # note: different form of build is added by has_one since # has_one refers to a single object, not to an # array-like object (eg user_infos) that can be # augmented with a build method </code></pre> <p>Rails 2.x introduced the autosave option for associations. I don't think it applies to the above (I'm using default). <a href="http://www.neeraj.name/2010/08/09/autosave-option-in-rails.html" rel="noreferrer">Autosave testing results.</a></p>
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    1. COThe child model in my example is called "user_info", when I try to do `user_info.create(:main => true)` it errors and says `undefined method \`create' for nil:NilClass`
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    2. COactually, the model is technically called `userInfo`
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    3. COTry `UserInfo.create`
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