Testing a lambda

Question

I am creating an import feature that imports CSV files into several tables. I made a module called `CsvParser` which parses a CSV file and creates records. My models that receive the create actions extends the`CsvParser`. They make a call to `CsvParser.create` and pass the correct attribute order and an optional lambda called `value_parser`. This lambda transforms values in a hash to a preffered format. ``` class Mutation < ActiveRecord::Base extend CsvParser def self.import_csv(csv_file) attribute_order = %w[reg_nr receipt_date reference_number book_date is_credit sum balance description] value_parser = lambda do |h| h["is_credit"] = ((h["is_credit"] == 'B') if h["is_credit"].present?) h["sum"] = -1 * h["sum"].to_f unless h["is_credit"] return [h] end CsvParser.create(csv_file, self, attribute_order, value_parser) end end ``` The reason that I'm using a lambda instead of checks inside the `CsvParser.create` method is because the lambda is like a business rule that belongs to this mod…

Accepted Answer

Some interesting questions. A couple of notes:

- You probably shouldn't have a return within the lambda. Just make the last statement [h].

If I understand the code correctly, the first and second lines of your lambda are overcomplicated. Reduce them to make them more readable and easier to refactor:

```
h["is_credit"] = (h['is_credit'] == 'B') # I *think* that will do the same
h['sum'] = h['sum'].to_f   # Your original code would have left this a string
h['sum'] *= -1 unless h['is_credit']
```

It looks like your lambda doesn't depend on anything external (aside from `h`), so I would test it separately. You could even make it a constant:

```
class Mutation < ActiveRecord::Base
  extend CsvParser    # <== See point 5 below

PARSE_CREDIT_AND_SUM = lambda do |h|
    h["is_credit"] = (h['is_credit'] == 'B') 
    h['sum'] = h['sum'].to_f
    h['sum'] *= -1 unless h['is_credit']
    [h]
  end
```

Without knowing the rationale, it's hard to say where you should put this code. My gut instinct is that it is not the job of the CSV parser (although a good parser may detect floating point numbers and convert them from strings?) Keep your CSV parser reusable. (Note: Re-reading, I think you've answered this question yourself - it is business logic, tied to the model. Go with your gut!)

Lastly, you are defining and the method `CsvParser.create`. You don't need to extend CsvParser to get access to it, although if you have other facilities in CsvParser, consider making `CsvParser.create` a normal module method called something like create_from_csv_file

Testing a lambda

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