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What Rails models are and how they organize database data structures

In relational databases, there are several different types of relationships different models can have:

  1. has_many for when there’s several instances of another model related to one of the selected model
  2. belongs_to for when one instance of this model can belong to several other instances of another model
    • Can also have polymorphic: true set if it belongs to multiple different models. A polymorphic association must be declared in both cases to complete it, such as in this example.
  class TeachingAssistant < ActiveRecord::Base
    belongs_to :ta_duty, polymorphic: true

  class SectionA < ActiveRecord::Base
    has_many :teaching_assistants, as: :ta_duty


You can also define methods within models to better manage the data that’s returned. For example, if one column returns a boolean that needs to be turned into a related string, the function can do that:

  # In the model:
  def label
    if model.boolean "First Label" else "Second Label" end

  # After calling an instance of the model:

Models use ActiveRecord to pull records from the database, and there’s several methods to use for getting it.

    .where(expense: @type)
    .where(expenses: { created_at: @start_date..@end_date })