我应该如何设计/构建数据库和ActiveRecord关联？ has_many：通过吗？

Question

Using Rails 4 (and Postgres), I'm trying to work out the best way in which to structure my ActiveRecord associations and corresponding database tables/relationships for a diet tracking app.

I want my app to be structured as follows: I have Document and FoodEntry models. Each Document has a number of FoodEntries . I want to be able to iterate over them like document.food_entries.each ... (which is easy with your typical has_many association).

However all the FoodEntries for each Document need to be able (potentially but not necessarily) to be subdivided by day , as this is a natural division for which logic and calculations must be able to be performed, in addition to doing them for the whole document. For instance I'd be using something like document.day(1).food_entries.each ... .

Furthermore, each day should be able to be subdivided (again, optionally) into meals in a similar manner, eg document.day(1).meal(1).food_entries.each ...

Lastly, there must be a way to record the user-specified order that the FoodEntries , meals, and days are in for each document. Presumably using number sequence(s)?

I was thinking there are a few ways I could do this:

1. Use a simple has_many relationship. Have day , meal and sort columns in the food_entries table, where the value for day and meal is left blank or given a default value if a day/meal isn't provided. Use a logic-based approach to get and sort the entries for a day or meal.

   Outline:

    class Document has_many :food_entries class FoodEntry belongs_to :document 

   Potential issues:

   • This might leave things a bit messy in general in the table?
   • All the logic for subdividing things would have to be hand-coded.
   • Storing/using the user-defined (ie arbitrary) sort order might get a bit complicated? The order for entries AND days AND meals would have to stored in and inferred from one sequence (unless more columns were added).
     
     

2. Use has_many :through to set up associations through days and meals (naming?) tables. Entries where a day/meal isn't specified get given a default. Both these tables have their own individual sort column, along with the food_entries table.

   Outline:

    class Document has_many :days has_many :meals, through: :days has_many :food_entries, through: :days (AND :meals???) class Day belongs_to :document has_many :meals has_many :food_entries, through: :meals class Meal belongs_to :day has_many :food_entries class FoodEntry belongs_to :meal 

   Potential issues:

   • Adds unnecessary relational complexity? (consider that days or at the very least meals are meant to be optional)
   • Can I even use has_many :food_entries through: ... in my Document model if it would have to go through both tables?
     
     

3. A compromise between the two approaches above: have a days table but keep meal in a column in the food_entries table.

4. Something else? Polymorphic association(s)?

This is getting a bit complicated to wrap my head around, and so I'm really having a hard time working out what I should use. What is the correct way to go about things?

---

A couple of final questions which are related but completely optional:

• Ideally the day value could be either a datetime value or an arbitrary string, depending on what the user sets. Is this possible?
• Could anyone point me to a resource that can inform me about sorting/ordering strategies? Like I said I assume the simplest way is to use a sequence of numbers, but I'm not exactly sure how I would work with such a sequence.

Answer 1

has_many :through

You'd only use a has_many :through relationship if you wanted to attribute multiple FoodEntries to Document , like this:

#app/models/document.rb
Class Document < ActiveRecord::Base
    has_many :food_entries_types
    has_many :food_entries, through: :food_entries_types
end

#app/models/food_entry_type.rb
Class FoodEntryType < ActiveRecord::Base
    belongs_to :document
    belongs_to :food_entry
end

#app/models/food_entry.rb
Class FoodEntry < ActiveRecord::Base
    has_many :food_entries_types
    has_many :documents, through: :food_entries_types
end

This would only allow you to associate many food_entries with a similar number of documents . Although you could add specific days & meals attributes to the join model, allowing you to call them as required

---

Scopes

I believe a much better option for you is to use ActiveRecord scopes :

#app/models/document.rb
Class Document < ActiveRecord::Base
    has_many :food_entries

    #uses [wday][3]
    #needs to return dates for specific day
    scope :day, ->(day = 1) { where(created_at: Date::DAYNAMES[day]) }
    scope :meal, ->(meal = 1) { where(meal: meal) }
end

Because scopes can be chained, I believe you'd be able to do this:

food = Document.day(1).meal(2).food_entries

---

Class Method

You could also create a class_method to achieve something similar:

#app/models/document.rb
Class Document < ActiveRecord::Base
    has_many :food_entries

    def self.sorted(day = 1, meal = 1)
        document = self.where("created_at = ? AND meal = ?", Date::DAYNAMES[day], meal)
    end
end

#app/controllers/documents_controller.rb
def show
    @document = Document.sorted
end

Answer 2

I ended up implementing my #2 option. This has given me the most flexibility and worked out well. I think it has been the most elegant approach for my use case.