OpenApiSpex.Schema.example/1 handles allOf/anyOf/oneOf too simplistically

[xxdavid]

Hi, first of all, thanks for this great library!

I've found issues with OpenApiSpex.Schema.example/1 when allOf/anyOf/oneOf fields are involved. Produced examples do not even pass validation/casting.

Looking at source code the source code of example/1, if it inputs a schema with one of these fields, it handles just one of them and ignores all other fields.

Here are two examples.

alias OpenApiSpex.Schema
schema =
  %Schema{
    type: :object,
    allOf: [
      %Schema{
        type: :object,
        properties: %{a: %Schema{type: :integer}},
        required: [:a]
      },
      %Schema{
        type: :object,
        properties: %{b: %Schema{type: :integer}},
        required: [:b]
      }
    ],
    oneOf: [
      %Schema{
        type: :object,
        properties: %{c: %Schema{type: :integer}},
        required: [:c]
      },
      %Schema{
        type: :object,
        properties: %{d: %Schema{type: :integer}},
        required: [:d]
      }
    ],
    properties: %{e: %Schema{type: :integer}},
    required: [:e]
  }

If I understand it correctly, this schema should describe an object that has both a and b and either c or d. But example/1 return just a map with c.

iex> example = OpenApiSpex.Schema.example(schema)
%{c: 0}
iex> Schema.cast(schema, example, %{})
{:error,
 [
   %OpenApiSpex.Cast.Error{
     reason: :all_of,
     value: %{c: 0},
     format: nil,
     type: nil,
     name: nil,
     path: [],
     length: 0,
     meta: %{invalid_schema: "object"}
   },
   %OpenApiSpex.Cast.Error{
     reason: :missing_field,
     value: %{c: 0},
     format: nil,
     type: nil,
     name: :a,
     path: [:a],
     length: 0,
     meta: %{}
   }
 ]}

Similar goes for the second example.

schema =
  %Schema{
    type: :object,
    allOf: [
      %Schema{
        type: :object,
        properties: %{a: %Schema{type: :integer}},
        required: [:a]
      },
      %Schema{
        type: :object,
        properties: %{b: %Schema{type: :integer}},
        required: [:b]
      }
    ],
    properties: %{c: %Schema{type: :integer}},
    required: [:c]
  }

This schema should describe an object with all three keys a, b, and c. However, example/1 return just a map with a and b.

iex> example = OpenApiSpex.Schema.example(schema)
%{a: 0, b: 0}
iex> Schema.cast(schema, example, %{})
{:error,
 [
   %OpenApiSpex.Cast.Error{
     reason: :missing_field,
     value: %{a: 0, b: 0},
     format: nil,
     type: nil,
     name: :c,
     path: [:c],
     length: 0,
     meta: %{}
   }
 ]}