Black Thursday Iteration 0 - Merchants & Items

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The goal of this iteration is to get the ball rolling by focusing on a “Data Access Layer” for multiple CSV files.

Data Access Layer

The idea of a DAL is to write classes which load and parse your raw data, allowing your system to then interact with rich Ruby objects to do more complex analysis. In this iteration, we’ll build the beginnings of a DAL by building the classes described below:

Merchant

The Merchant is one of the critical concepts in our data hierarchy.

• id - returns the integer id of the merchant
• name - returns the name of the merchant

We create an instance like this:

m = Merchant.new({:id => 5, :name => "Turing School"})
# => <Merchant...>

MerchantRepository

The MerchantRepository is responsible for holding and searching our Merchant instances. It offers the following methods:

• all - returns an array of all known Merchant instances
• find_by_id(id) - returns either nil or an instance of Merchant with a matching ID
• find_by_name(name) - returns either nil or an instance of Merchant having done a case insensitive search
• find_all_by_name(name) - returns either [] or one or more matches which contain the supplied name fragment, case insensitive
• create(attributes) - create a new Merchant instance with the provided attributes. The new Merchant’s id should be the current highest Merchant id plus 1.
• update(id, attributes) - update the Merchant instance with the corresponding id with the provided attributes. Only the merchant’s name attribute can be updated.
• delete(id) - delete the Merchant instance with the corresponding id

The data can be found in data/merchants.csv. The instance should be created in the SalesEngine (see below), so that the instance is created and used like this:

se = SalesEngine.from_csv({
  :items     => "./data/items.csv",
  :merchants => "./data/merchants.csv",
})

mr = se.merchants
merchant = mr.find_by_name("CJsDecor")
# => <Merchant...>

Item

The Item instance offers the following methods:

• id - returns the integer id of the item
• name - returns the name of the item
• description - returns the description of the item
• unit_price - returns the price of the item formatted as a BigDecimal
• created_at - returns a Time instance for the date the item was first created
• updated_at - returns a Time instance for the date the item was last modified
• merchant_id - returns the integer merchant id of the item

It also offers the following method:

• unit_price_to_dollars - returns the price of the item in dollars formatted as a Float

We create an instance like this:

i = Item.new({
  :id          => 1,
  :name        => "Pencil",
  :description => "You can use it to write things",
  :unit_price  => BigDecimal(10.99,4),
  :created_at  => Time.now,
  :updated_at  => Time.now,
  :merchant_id => 2
})
# => <Item...>

i.unit_price_to_dollars
# => 10.99

ItemRepository

The ItemRepository is responsible for holding and searching our Item instances. This object represents one line of data from the file items.csv.

It offers the following methods:

• all - returns an array of all known Item instances
• find_by_id(id) - returns either nil or an instance of Item with a matching ID
• find_by_name(name) - returns either nil or an instance of Item having done a case insensitive search
• find_all_with_description(description) - returns either [] or instances of Item where the supplied string appears in the item description (case insensitive)
• find_all_by_price(price) - returns either [] or instances of Item where the supplied price exactly matches
• find_all_by_price_in_range(range) - returns either [] or instances of Item where the supplied price is in the supplied range (a single Ruby range instance is passed in)
• find_all_by_merchant_id(merchant_id) - returns either [] or instances of Item where the supplied merchant ID matches that supplied
• create(attributes) - create a new Item instance with the provided attributes. The new Item’s id should be the current highest Item id plus 1.
• update(id, attributes) - update the Item instance with the corresponding id with the provided attributes. Only the item’s name, description, and unit_price attributes can be updated. This method will also change the items updated_at attribute to the current time.
• delete(id) - delete the Item instance with the corresponding id

Similar to the MerchantRepository, the ItemRepository is initialized from the SalesEngine class (see below), and its data can be found in data/items.csv. It is initialized and used like this:

se = SalesEngine.from_csv({
  :items     => "./data/items.csv",
  :merchants => "./data/merchants.csv"
})

ir   = se.items
item = ir.find_by_name("Item Repellat Dolorum")
# => <Item...>

SalesEngine

Once we have an ItemRepository and a MerchantRepository, we can tie everything together with one common root, a SalesEngine instance:

se = SalesEngine.from_csv({
  :items     => "./data/items.csv",
  :merchants => "./data/merchants.csv",
})

From there we can find the child instances:

• items returns an instance of ItemRepository with all the item instances loaded
• merchants returns an instance of MerchantRepository with all the merchant instances loaded