The counter cache method can be used to store the number of child items with ease.
My scheme of things has multiple users adding entries into the child table. Will the active record handle this? Will the number be updated accordingly if more than one user simultaneously add child entries?
No, the counter cache can be set incorrectly if children are added
concurrently. To be sure the counter_cache is updated correctly,
@parent = Parent.find( ..., :from => 'parents for update' )
@parent.children.create( params[:child] )
Similarly for child deletion.