How to use Jason.Encode

Questions / Help
1

Its not entirely clear from the documentation how to use Jason.Encode.

I have the following structure:

  %GoogleApi.Storage.V1.Model.Bucket{
    acl: nil,
    billing: nil,
    cors: nil,
    defaultEventBasedHold: false,
    defaultObjectAcl: nil,
    encryption: nil,
    etag: "CAE=",
    iamConfiguration: %GoogleApi.Storage.V1.Model.BucketIamConfiguration{
      bucketPolicyOnly: %GoogleApi.Storage.V1.Model.BucketIamConfigurationBucketPolicyOnly{
        enabled: false,
        lockedTime: nil
      },
      uniformBucketLevelAccess: %GoogleApi.Storage.V1.Model.BucketIamConfigurationUniformBucketLevelAccess{
        enabled: false,
        lockedTime: nil
      }
    },
    id: "fafdafdaf",
    kind: "storage#bucket",
    labels: nil,
    lifecycle: nil,
    location: "US",
    locationType: "multi-region",
    logging: nil,
    metageneration: "1",
    name: "fafdafdaf",
    owner: nil,
    projectNumber: "454545",
    retentionPolicy: nil,
    selfLink: "https://www.googleapis.com/storage/v1/b/fafdafdaf",
    storageClass: "STANDARD",
    timeCreated: ~U[2020-03-05 02:00:31.923Z],
    updated: ~U[2020-03-05 02:00:31.923Z],
    versioning: nil,
    website: nil
  },
  %GoogleApi.Storage.V1.Model.Bucket{
    acl: nil,
    billing: nil,
    cors: nil,
    defaultEventBasedHold: false,
    defaultObjectAcl: nil,
    encryption: nil,
    etag: "CAE=",
    iamConfiguration: %GoogleApi.Storage.V1.Model.BucketIamConfiguration{
      bucketPolicyOnly: %GoogleApi.Storage.V1.Model.BucketIamConfigurationBucketPolicyOnly{
        enabled: false,
        lockedTime: nil
      },
      uniformBucketLevelAccess: %GoogleApi.Storage.V1.Model.BucketIamConfigurationUniformBucketLevelAccess{
        enabled: false,
        lockedTime: nil
      }
    },
    id: "gcs_bucket_api",
    kind: "storage#bucket",
    labels: nil,
    lifecycle: nil,
    location: "US",
    locationType: "multi-region",
    logging: nil,
    metageneration: "1",
    name: "gcs_bucket_api",
    owner: nil,
    projectNumber: "454545",
    retentionPolicy: nil,
    selfLink: "https://www.googleapis.com/storage/v1/b/gcs_bucket_api",
    storageClass: "STANDARD",
    timeCreated: ~U[2020-03-05 01:34:16.147Z],
    updated: ~U[2020-03-05 01:34:16.147Z],
    versioning: nil,
    website: nil
  }
]

I would like to give the whole thing as JSON but it says I need to implement it using @derive Jason.Encode. But I don’t understand how that would be done given the example from documentation adding the following did not help:

"Jason.Encoder protocol must always be explicitly implemented.\n\nIf you own the struct, you can derive the implementation specifying which fields should be encoded to JSON:\n\n @derive {Jason.Encoder, only: [....]}\n defstruct ...\n\nIt is also possible to encode all fields, although this should be used carefully to avoid accidentally leaking private information when new fields are added:\n\n @derive Jason.Encoder\n defstruct ...\n\nFinally, if you don't own the struct you want to encode to JSON, you may use Protocol.derive/3 placed outside of any module:\n\n Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])\n

Given the following:

{:ok, token} = Goth.Token.for_scope("https://www.googleapis.com/auth/cloud-platform")
conc = GoogleApi.Storage.V1.Connection.new(token.token)
{:ok, response} = GoogleApi.Storage.V1.Api.Buckets.storage_buckets_list(conc, "PROJECT")

What is the right way to make response.items which is a list of structs with nested structs into JSON using Jason?

Solution I came up without Jason works great, but would be cleaner if I used Jason to encode my structs?

data = Enum.map(response.items, fn x -> 
        x = Map.update!(x, :iamConfiguration, &(Map.from_struct(&1)) )
        x_iamConf = Map.get(x, :iamConfiguration)
    
        x = Map.update!(x, :iamConfiguration, fn x_iamConf -> 
            x_iamConf = Map.update!(x_iamConf, :bucketPolicyOnly, &(Map.from_struct(&1))) 
            x_iamConf = Map.update!(x_iamConf, :uniformBucketLevelAccess, &(Map.from_struct(&1))) 
        end)
    
        x = Map.from_struct(x)
    end)
    json(conn, data)
2

put @derive {Jason.Encoder} in each module just before the struct is defined. You can use the :only and :except options to define which items in your struct are encoded, e.g.

defmodule A do
  @derive {Jason.Encode, except[:foo]}
  defstruct [:foo, :bar, :baz]
end

will result in bar & baz being encoded

3 Likes
3

Can you give a more concrete example based on the data structure I provided?

I’m not defining that struct, it’s provided by the google storage api.

I also tried using the Protocol method it mentions in the docs but it gave the same not implemented error.

4

Man, I’m still stuck on using this library.

defmodule GcsBucket do
  use GcsBucketApiWeb, :controller

  alias Jason.{EncodeError, Encoder}

  def index(conn, _params) do
    {:ok, token} = Goth.Token.for_scope("https://www.googleapis.com/auth/cloud-platform")
    conc = GoogleApi.Storage.V1.Connection.new(token.token)
    {:ok, response} = GoogleApi.Storage.V1.Api.Buckets.storage_buckets_list(conc, "test")
    #{_, data2} = Jason.encode(%{acl: "A", billing: "B", id: "C"})
    {_, data2} = Jason.encode(response.items)
    text(conn, data2)
  end

  defmodule GoogleApi.Storage.V1.Model.Bucket do
    @derive {Jason.Encoder, only: [:acl, :billing, :id]}
    defstruct [:acl, :billing, :id]
  end

  defimpl Jason.Encoder, for: [GoogleApi.Storage.V1.Model.Bucket] do
    def encode(list, opts) do
      %{
        acl: "FOO",
        billing: "BAR", 
        id: "BAZ"
       }
    end
  end
end

It keeps crashing before it

1 Like
5

Found the solution. I miss Poision. Don’t understand why it works this way though…I’ll have to read about the Protocol library I guess. Thanks for the help though.

defmodule GcsBucketApiWeb.BucketController do
  use GcsBucketApiWeb, :controller
  require Protocol
  Protocol.derive(Jason.Encoder, GoogleApi.Storage.V1.Model.Bucket)
  Protocol.derive(Jason.Encoder, GoogleApi.Storage.V1.Model.BucketIamConfiguration)
  Protocol.derive(Jason.Encoder, GoogleApi.Storage.V1.Model.BucketIamConfigurationBucketPolicyOnly)
  Protocol.derive(Jason.Encoder, GoogleApi.Storage.V1.Model.BucketIamConfigurationUniformBucketLevelAccess)

  alias Jason.{EncodeError, Encoder}


  def index(conn, _params) do
    {:ok, token} = Goth.Token.for_scope("https://www.googleapis.com/auth/cloud-platform")
    conc = GoogleApi.Storage.V1.Connection.new(token.token)
    {:ok, response} = GoogleApi.Storage.V1.Api.Buckets.storage_buckets_list(conc, "test")

    {_, data} = Jason.encode(response.items)
    text(conn, data)
  end
end
1 Like
6

I was just writing up a small test case - yes, the answer is in the Jason doco guide just before the license. Here’s the minimal test case to represent your problem:

# Modules we don't have code for
defmodule A do
  defstruct [:foo, :bar, :baz]
end

defmodule B do
  defstruct [:a, :y, :z]
end

# We write this code outside of a module definition
require Protocol

Protocol.derive(Jason.Encoder, A)
Protocol.derive(Jason.Encoder, B)

defmodule Blah do
  def test do
    a = %A{foo: "foo", bar: "bar", baz: 2}
    b = %B{a: a, y: "Why?"}

    Jason.encode!(b)

    # Gives use the expected result:
    "{\"a\":{\"bar\":\"bar\",\"baz\":2,\"foo\":\"foo\"},\"y\":\"Why?\",\"z\":null}"
  end
end
1 Like
7

Sometimes it is simpler to just use Map.from_struct/1 before passing data to Jason :slight_smile:

1 Like