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I am trying to poll the GitHub API for issues and print them out. To do so, I need to deserialize a nested JSON structure that I receive from a cURL GET request.

I am trying to get the url for all the objects in the items array:

{ "total_count": 4905, "incomplete_results": false, "items": [ { "url": "https://api.github.com/repos/servo/saltfs/issues/789", "repository_url": "https://api.github.com/repos/servo/saltfs", "labels_url": "https://api.github.com/repos/servo/saltfs/issues/789/labels{/name}", "comments_url": "https://api.github.com/repos/servo/saltfs/issues/789/comments", "events_url": "https://api.github.com/repos/servo/saltfs/issues/789/events", "html_url": "https://github.com/servo/saltfs/issues/789", "id": 293260512, "number": 789, "title": "Stop setting $CARGO_HOME to its default value", "user": { "login": "SimonSapin", "id": 291359, "avatar_url": "https://avatars0.githubusercontent.com/u/291359?v=4", "gravatar_id": "", "url": "https://api.github.com/users/SimonSapin", "html_url": "https://github.com/SimonSapin", "followers_url": "https://api.github.com/users/SimonSapin/followers", "following_url": "https://api.github.com/users/SimonSapin/following{/other_user}", "gists_url": "https://api.github.com/users/SimonSapin/gists{/gist_id}", "starred_url": "https://api.github.com/users/SimonSapin/starred{/owner}{/repo}", "subscriptions_url": "https://api.github.com/users/SimonSapin/subscriptions", "organizations_url": "https://api.github.com/users/SimonSapin/orgs", "repos_url": "https://api.github.com/users/SimonSapin/repos", "events_url": "https://api.github.com/users/SimonSapin/events{/privacy}", "received_events_url": "https://api.github.com/users/SimonSapin/received_events", "type": "User", "site_admin": false }, "labels": [ { "id": 341722396, "url": "https://api.github.com/repos/servo/saltfs/labels/E-easy", "name": "E-easy", "color": "02e10c", "default": false } ], "state": "open", "locked": false, "assignee": null, "assignees": [ ], "milestone": null, "comments": 0, "created_at": "2018-01-31T18:16:09Z", "updated_at": "2018-01-31T18:16:49Z", "closed_at": null, "author_association": "MEMBER", "body": "In `buildbot/master/files/config/environments.py` we set `CARGO_HOME` to Cargo’s default value. Now that `mach` does not set it (since https://github.com/servo/servo/pull/19395), this has no effect. We can remove these lines.", "score": 1.0 }, { "url": "https://api.github.com/repos/servo/servo/issues/19916", "repository_url": "https://api.github.com/repos/servo/servo", "labels_url": "https://api.github.com/repos/servo/servo/issues/19916/labels{/name}", "comments_url": "https://api.github.com/repos/servo/servo/issues/19916/comments", "events_url": "https://api.github.com/repos/servo/servo/issues/19916/events", "html_url": "https://github.com/servo/servo/issues/19916", "id": 293237180, "number": 19916, "title": "Use a macro to create null-terminated C strings", "user": { "login": "jdm", "id": 27658, "avatar_url": "https://avatars1.githubusercontent.com/u/27658?v=4", "gravatar_id": "", "url": "https://api.github.com/users/jdm", "html_url": "https://github.com/jdm", "followers_url": "https://api.github.com/users/jdm/followers", "following_url": "https://api.github.com/users/jdm/following{/other_user}", "gists_url": "https://api.github.com/users/jdm/gists{/gist_id}", "starred_url": "https://api.github.com/users/jdm/starred{/owner}{/repo}", "subscriptions_url": "https://api.github.com/users/jdm/subscriptions", "organizations_url": "https://api.github.com/users/jdm/orgs", "repos_url": "https://api.github.com/users/jdm/repos", "events_url": "https://api.github.com/users/jdm/events{/privacy}", "received_events_url": "https://api.github.com/users/jdm/received_events", "type": "User", "site_admin": false }, "labels": [ { "id": 89384911, "url": "https://api.github.com/repos/servo/servo/labels/C-assigned", "name": "C-assigned", "color": "02d7e1", "default": false }, { "id": 15997664, "url": "https://api.github.com/repos/servo/servo/labels/E-easy", "name": "E-easy", "color": "02e10c", "default": false }, { "id": 135307111, "url": "https://api.github.com/repos/servo/servo/labels/I-cleanup", "name": "I-cleanup", "color": "e11d21", "default": false } ], "state": "open", "locked": false, "assignee": null, "assignees": [ ], "milestone": null, "comments": 3, "created_at": "2018-01-31T17:04:06Z", "updated_at": "2018-01-31T22:03:56Z", "closed_at": null, "author_association": "MEMBER", "body": "When we write them by hand (eg. `b\"some string\\0\"`), we invariably get them wrong in ways that are tricky to notice (https://github.com/servo/servo/pull/19915). We should use a macro like this instead:\r\n```rust\r\nmacro_rules! c_str {\r\n ($str:expr) => {\r\n concat!($str, \"\\0\").as_bytes()\r\n }\r\n}\r\n```\r\nThis would allow us to write code like `(c_str!(\"PEParseDeclarationDeclExpected\"), Action::Skip)` instead of https://github.com/emilio/servo/blob/d82c54bd3033cc3277ebeb4854739bebe4e20f2f/ports/geckolib/error_reporter.rs#L237. We should be able to clean up all of the uses in that file.\r\n\r\nNo need to run any automated tests; if it builds with `./mach build-geckolib`, then it's good enough for a pull request.", "score": 1.0 } ]}

My request function makes a cURL request and receives the above JSON. I then use serde_json to deserialize the JSON

main.rs

extern crate serde; #[macro_use] extern crate serde_derive; extern crate serde_json; mod engine; mod server; use engine::request; use std::string::String; use self::serde_json::{Error, Value}; #[derive(Serialize, Deserialize)] struct obj { items: Vec<String>, } fn main() { let output_jn: String = request( "https://api.github.com/search/issues?q=is:issue+label:e-easy", ).to_string(); //gets json structure as string let json: obj = serde_json::from_str(&output_jn).unwrap(); for elem in json.iter() { println!("{:?}", elem); } }

I get the following error message

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ErrorImpl { code: Message("invalid type: map, expected a sequence"), line: 1, column: 0 }', libcore/result.rs:945:5 note: Run with `RUST_BACKTRACE=1` for a backtrace.

I'm certain I'm making a stupid mistake in deserializing my JSON structure, I've tried a number of permutations and combinations but I couldn't get anything to work.

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  • let json: Vec<String> = serde_json::from_str — Why are you deserializing that JSON object as a vector? Why are you never using obj (which should be Obj)? Commented Feb 3, 2018 at 15:00
  • Thats a typo i was trying out something i saw online, I was using struct obj and getting this error. Edited the code above. Commented Feb 3, 2018 at 16:55
  • 1
    Ok, then the question becomes: Why are you deserializing that JSON array of objects as an array of strings? Commented Feb 3, 2018 at 17:11
  • I guess thats my confusion is what is the correct data structure for deserializing should it be items: Vec<Vec<String>> ? Commented Feb 4, 2018 at 3:47

1 Answer 1

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35

Have a look at this part of your JSON input data:

{ ... "items": [ { ... "title": "Stop setting $CARGO_HOME to its default value", ... } ] }
  • The top-level data structure is a JSON map, so in Rust this will be represented as a struct. I will use your name Obj.
  • The top-level JSON map has a key called "items" so in Rust this will be a field items inside the Obj struct.
  • The value of "items" in the map is a JSON array, so in Rust let's use a Vec.
  • Each element in the JSON array is a JSON map so in Rust we need a struct for those. We can call it Issue.
  • Each issue has a JSON key called "title" so this will be a field title inside the Issue struct.
  • The value of "title" is a JSON string so we can use Rust's String type for the field.
#[derive(Deserialize, Debug)] struct Obj { items: Vec<Issue>, } #[derive(Deserialize, Debug)] struct Issue { title: String, } fn main() { let j = /* get the JSON data */; let issues = serde_json::from_str::<Obj>(j).unwrap(); for i in issues.items { println!("{:#?}", i); } }
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2 Comments

> The top-level data structure is a JSON map, so in Rust this will be represented as a struct < This doesn't need to be a struct - can be done with eg HashMap
@til My experiments don't bear that out, if there are keys other than "items" in the top-level (outer) dictionary/map here, i.e. keys with a value which is something other than struct Issue, which there are in the example given by the OP: "total_count": integer of some kind, and "incomplete_results": bool. I then get an Error. Whereas if everything is covered by a struct the other keys are just ignored.

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