First README draft

README.md

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+# EntityUtils
+
+EntityUtils gives you tools to add a schemas to your data structures, namely Ruby hashes. It provides you means to perform validations and transformations to the hash structures.
+
+## Motivation
+
+Ruby is dynamically typed language. One of the drawbacks of this is that by looking your method signature, you can not be sure about the type and the structure of the parameters. For simple methods and simple types such as number, string, booleans etc. this may not be big issue, but as soon as you start passing hashes to methods, you easily loose track what data should be inside the hash.
+
+## Installation
+
+With bundler:
+
+```ruby
+# Add to Gemfile
+
+gem 'entity_utils', git: 'https://github.com/sharetribe/entity-utils.git'
+```
+
+## Usage
+
+You start by defining a builder, which will build the Entity hashes.
+
+The builder takes the schema as a parameter. Each field specification in the schema is represented as an array.
+
+For example, the following array describes a "name" field, which needs to be present (mandatory validator) and it needs to be string:
+
+```ruby
+[:name, :string, :mandatory]
+```
+
+A complete schema is a collection of these field specifications.
+
+The following example defines a builder for Person entity:
+
+```ruby
+Person = EntityUtils.define_builder(
+  [:name, :string, :mandatory],
+  [:sex, :to_symbol, one_of: [:f, :m]]
+)
+
+Person.call({
+  name: "John",
+  sex: "m",
+  age: 30
+})
+
+#=> { name: "John", sex: :m }
+```
+
+The Person builder schema contains to fields, `name` and `sex`. `name` needs to be present and it needs to be string. `sex` is first transformed to symbol and then validated that it is either `:m` of `:f`. `age` is ignored because it's not part of the schema.
+
+## Transformers
+
+Transformers transform the content of the hash. They are run before validators.
+
+### List of predefined transformers
+
+TODO
+
+### Custom transformers
+
+You can also define custom transformers. The custom transformer needs to be a lambda that returns ???. In addition, the transformer needs to be idempotent, which is a fancy way of saying that running the transformer once should return the same result as running it twice.
+
+TODO Improve this
+
+```ruby
+
+to_time = ->(v) {
+  if (v.is_a? Time) {
+    # This is important check to ensure idempotency
+    v
+  } elsif (v.nil?) {
+    v
+  } else {
+    Time.at(v)
+  }
+}
+
+[:time, transform_with: to_time]
+```
+
+## Validators
+
+Validators validate the content of the hash.
+
+Validators are run:
+
+* Before after transformers
+* In the order they are defined
+
+### Predefined validators
+
+TODO
+
+### Custom validators
+
+You can also define custom validators. The custom validator needs to be a lambda that returns ???
+
+TODO Improve this
+
+```ruby
+
+even_number = ->(v) { v.nil? && v.even? }
+
+[:even_number, validate_with: even_number]
+```
+
+### Receive validation errors instead of exceptions
+
+### Skip validations for production
+
+Skipping validations in production will give you a slight performance gain. You can do this by adding an initializer:
+
+```ruby
+# config/initializer/entity_utils.rb
+
+if Rails.env == "production"
+  EntityUtils.configure!(
+    validate: false
+  )
+end
+```
+
+Transformers will be run even if validations are skipped:
+
+```ruby
+EntityUtils.configure!(
+   validate: false
+)
+
+person = EntityUtils.define_builder(
+  [:name, :string, :mandatory],
+  [:sex, :to_symbol, one_of: [:f, :m]]
+)
+
+person.call(name: "John", sex: "male")
+
+#=> {name: "John", sex: :male}
+```
+
+If you want to make sure that validations are always run (despite the global configuration), use the `define_builder_validate_always` method:
+
+```ruby
+EntityUtils.configure!(
+   validate: false
+)
+
+person = EntityUtils.define_builder_validate_always(
+  [:name, :string, :mandatory],
+  [:sex, :to_symbol, one_of: [:f, :m]]
+)
+
+person.call(name: "John", sex: "male")
+
+#=> Exception
+```
+
+## Nested entities
+
+You can also nest entities.
+
+Use `entity` to nest a single entity:
+
+```ruby
+
+Address = EntityUtils.define_builder(
+  [:street, :string, :mandatory],
+  [:city, :string, :mandatory],
+  [:postal_code, :string, :mandatory]
+)
+
+Person = EntityUtils.define_builder(
+  [:name, :string, :mandatory],
+  [:address, entity: Address]
+)
+
+Person.call({
+  name: "John",
+  address: {
+    street: "Bulevardi 14",
+    city: "Helsinki",
+    postal_code: "00100"
+  }
+})
+```
+
+Use `collection` to nest a collection of entities:
+
+```ruby
+Item = EntityUtils.define_builder(
+  [:name, :string, :mandatory],
+  [:price_cents, :fixnum, default: 0]
+)
+
+ShoppingCart = EntityUtils.define_builder(
+  [:items, collection: Item]
+)
+
+cart = ShoppingCart.new(
+  items: [
+    {name: "Shirt", price_cents: 5000},
+    {name: "Shoes", price_cents: 7500},
+    {name: "Free delivery!"}
+  ]
+)
+```
+
+## Inspired by
+
+pluratic/schema
+
+## Sponsored by
+
+Sharetribe - An Open Source platrom for creating marketplaces
+
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