Ruby on Rails/ActiveRecord

ActiveRecord - The Model

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See the way the models are named and what names to avoid
How to get your data inside the database in a convenient way
Connect different models together
Read about how to manipulate your data at certain states of their existence
Check your input for their correctness
How to look up different criteria without writing SQL
Write advanced aggregations inside your model
Do basic mathematical calculations with your data
Extend functionality with acts_as