Adds additional associations dynamically from app config.

Reads config for the given OTP application, under the name of the current module. Each key maps to an Ecto.Schema function:

• belongs_to
• field
• has_many
• has_one
• many_to_many

Each of these keys should map to a keyword list where the key is the name of the field or association and the value is one of:

• A type
• A tuple of type and options (keyword list)

Example Schema:

defmodule My.Schema do
  use Ecto.Schema
  import Exto, only: [flex_schema: 1]

  schema "my_table" do
    field :name, :string # just normal schema things
    flex_schema(:my_app) # boom! give me the stuff
  end
end

Example configuration:

config :my_app, My.Schema,
  belongs_to: [
    foo: Foo,                   # belongs_to :foo, Foo
    bar: {Bar, type: :integer}, # belongs_to :bar, Bar, type: :integer
  ],
  field: [
    foo: :string,                # field :foo, :string
    bar: {:integer, default: 4}, # field :foo, :integer, default: 4
  ],
  has_one: [
    foo: Foo,                             # has_one :foo, Foo
    bar: {Bar, foreign_key: :the_bar_id}, # has_one :bar, Bar, foreign_key: :the_bar_id
  ]
  has_many: [
    foo: Foo,                             # has_many :foo, Foo
    bar: {Bar, foreign_key: :the_bar_id}, # has_many :bar, Bar, foreign_key: :the_bar_id
  ]
  many_to_many: [
    foo: Foo,                         # many_to_many :foo, Foo
    bar: {Bar, join_through: FooBar}, # many_to_many :bar, Bar, :join_through: FooBar
  ]

This one won't work very well because we define foo and bar 5 times each, but I think you get the point.

Reading of configuration is done during compile time. The relations will be baked in during compilation, thus:

• Do not expect this to work in runtime config.
• You will need to rebuild all dependencies which use this macro when you change their configuration.