diff --git a/Cargo.toml b/Cargo.toml
index 3f2732444..a9378adc4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["milli", "filter-parser", "http-ui", "benchmarks", "infos", "helpers", "cli"]
+members = ["milli", "filter-parser", "flatten-serde-json", "http-ui", "benchmarks", "infos", "helpers", "cli"]
 default-members = ["milli"]
 
 [profile.dev]
diff --git a/flatten-serde-json/Cargo.toml b/flatten-serde-json/Cargo.toml
new file mode 100644
index 000000000..db92c1ded
--- /dev/null
+++ b/flatten-serde-json/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "flatten-serde-json"
+version = "0.1.0"
+edition = "2021"
+description = "Flatten serde-json objects like elastic search"
+readme = "README.md"
+author = ["Tamo tamo@meilisearch.com"]
+repository = "https://github.com/irevoire/flatten-serde-json"
+keywords = ["json", "flatten"]
+categories = ["command-line-utilities"]
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+serde_json = "1.0"
diff --git a/flatten-serde-json/README.md b/flatten-serde-json/README.md
new file mode 100644
index 000000000..a1dd7d275
--- /dev/null
+++ b/flatten-serde-json/README.md
@@ -0,0 +1,153 @@
+# Flatten serde Json
+
+This crate flatten [`serde_json`](https://docs.rs/serde_json/latest/serde_json/) `Object` in a format
+similar to [elastic search](https://www.elastic.co/guide/en/elasticsearch/reference/current/nested.html).
+
+## Examples
+
+### There is nothing to do
+
+```json
+{
+  "id": "287947",
+  "title": "Shazam!",
+  "release_date": 1553299200,
+  "genres": [
+    "Action",
+    "Comedy",
+    "Fantasy"
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "id": "287947",
+  "title": "Shazam!",
+  "release_date": 1553299200,
+  "genres": [
+    "Action",
+    "Comedy",
+    "Fantasy"
+  ]
+}
+```
+
+------------
+
+### Objects
+
+```json
+{
+  "a": {
+    "b": "c",
+    "d": "e",
+    "f": "g"
+  }
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": "c",
+  "a.d": "e",
+  "a.f": "g"
+}
+```
+
+------------
+
+### Array of objects
+
+```json
+{
+  "a": [
+    { "b": "c" },
+    { "b": "d" },
+    { "b": "e" },
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": ["c", "d", "e"],
+}
+```
+
+------------
+
+### Array of objects with normal value in the array
+
+```json
+{
+  "a": [
+    42,
+    { "b": "c" },
+    { "b": "d" },
+    { "b": "e" },
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a": 42,
+  "a.b": ["c", "d", "e"],
+}
+```
+
+------------
+
+### Array of objects of array of objects of ...
+
+```json
+{
+  "a": [
+    "b",
+    ["c", "d"],
+    { "e": ["f", "g"] },
+    [
+        { "h": "i" },
+        { "e": ["j", { "z": "y" }] },
+    ],
+    ["l"],
+    "m",
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a": ["b", "c", "d", "l", "m"],
+  "a.e": ["f", "g", "j"],
+  "a.h": "i",
+  "a.e.z": "y",
+}
+```
+
+------------
+
+### Collision between a generated field name and an already existing field
+
+```json
+{
+  "a": {
+    "b": "c",
+  },
+  "a.b": "d",
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": ["c", "d"],
+}
+```
+
diff --git a/flatten-serde-json/fuzz/Cargo.toml b/flatten-serde-json/fuzz/Cargo.toml
new file mode 100644
index 000000000..2e0510d5f
--- /dev/null
+++ b/flatten-serde-json/fuzz/Cargo.toml
@@ -0,0 +1,26 @@
+[package]
+name = "flatten_serde_json-fuzz"
+version = "0.0.0"
+authors = ["Automatically generated"]
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+arbitrary-json = "0.1.1"
+
+[dependencies.flatten_serde_json]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "flatten"
+path = "fuzz_targets/flatten.rs"
+test = false
+doc = false
diff --git a/flatten-serde-json/fuzz/fuzz_targets/flatten.rs b/flatten-serde-json/fuzz/fuzz_targets/flatten.rs
new file mode 100644
index 000000000..399d1c484
--- /dev/null
+++ b/flatten-serde-json/fuzz/fuzz_targets/flatten.rs
@@ -0,0 +1,8 @@
+#![no_main]
+use arbitrary_json::ArbitraryObject;
+use flatten_serde_json::flatten;
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|object: ArbitraryObject| {
+    let _ = flatten(&object);
+});
diff --git a/flatten-serde-json/src/lib.rs b/flatten-serde-json/src/lib.rs
new file mode 100644
index 000000000..734ae2a24
--- /dev/null
+++ b/flatten-serde-json/src/lib.rs
@@ -0,0 +1,264 @@
+#![doc = include_str!("../README.md")]
+
+use serde_json::{json, Map, Value};
+
+pub fn flatten(json: &Map<String, Value>) -> Map<String, Value> {
+    let mut obj = Map::new();
+    insert_object(&mut obj, None, json);
+    obj
+}
+
+fn insert_object(
+    base_json: &mut Map<String, Value>,
+    base_key: Option<&str>,
+    object: &Map<String, Value>,
+) {
+    for (key, value) in object {
+        let new_key = base_key.map_or_else(|| key.clone(), |base_key| format!("{base_key}.{key}"));
+
+        if let Some(array) = value.as_array() {
+            insert_array(base_json, &new_key, array);
+        } else if let Some(object) = value.as_object() {
+            insert_object(base_json, Some(&new_key), object);
+        } else {
+            insert_value(base_json, &new_key, value.clone());
+        }
+    }
+}
+
+fn insert_array(base_json: &mut Map<String, Value>, base_key: &str, array: &Vec<Value>) {
+    for value in array {
+        if let Some(object) = value.as_object() {
+            insert_object(base_json, Some(base_key), object);
+        } else if let Some(sub_array) = value.as_array() {
+            insert_array(base_json, base_key, sub_array);
+        } else {
+            insert_value(base_json, base_key, value.clone());
+        }
+    }
+}
+
+fn insert_value(base_json: &mut Map<String, Value>, key: &str, to_insert: Value) {
+    debug_assert!(!to_insert.is_object());
+    debug_assert!(!to_insert.is_array());
+
+    // does the field aleardy exists?
+    if let Some(value) = base_json.get_mut(key) {
+        // is it already an array
+        if let Some(array) = value.as_array_mut() {
+            array.push(to_insert);
+        // or is there a collision
+        } else {
+            let value = std::mem::take(value);
+            base_json[key] = json!([value, to_insert]);
+        }
+        // if it does not exist we can push the value untouched
+    } else {
+        base_json.insert(key.to_string(), json!(to_insert));
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn no_flattening() {
+        let mut base: Value = json!({
+          "id": "287947",
+          "title": "Shazam!",
+          "release_date": 1553299200,
+          "genres": [
+            "Action",
+            "Comedy",
+            "Fantasy"
+          ]
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        println!(
+            "got:\n{}\nexpected:\n{}\n",
+            serde_json::to_string_pretty(&flat).unwrap(),
+            serde_json::to_string_pretty(&json).unwrap()
+        );
+
+        assert_eq!(flat, json);
+    }
+
+    #[test]
+    fn flatten_object() {
+        let mut base: Value = json!({
+          "a": {
+            "b": "c",
+            "d": "e",
+            "f": "g"
+          }
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a.b": "c",
+                "a.d": "e",
+                "a.f": "g"
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+
+    #[test]
+    fn flatten_array() {
+        let mut base: Value = json!({
+          "a": [
+            { "b": "c" },
+            { "b": "d" },
+            { "b": "e" },
+          ]
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a.b": ["c", "d", "e"],
+            })
+            .as_object()
+            .unwrap()
+        );
+
+        // here we must keep 42 in "a"
+        let mut base: Value = json!({
+          "a": [
+            42,
+            { "b": "c" },
+            { "b": "d" },
+            { "b": "e" },
+          ]
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a": 42,
+                "a.b": ["c", "d", "e"],
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+
+    #[test]
+    fn collision_with_object() {
+        let mut base: Value = json!({
+          "a": {
+            "b": "c",
+          },
+          "a.b": "d",
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a.b": ["c", "d"],
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+
+    #[test]
+    fn collision_with_array() {
+        let mut base: Value = json!({
+          "a": [
+            { "b": "c" },
+            { "b": "d", "c": "e" },
+            [35],
+          ],
+          "a.b": "f",
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a.b": ["c", "d", "f"],
+                "a.c": "e",
+                "a": 35,
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+
+    #[test]
+    fn flatten_nested_arrays() {
+        let mut base: Value = json!({
+          "a": [
+            ["b", "c"],
+            { "d": "e" },
+            ["f", "g"],
+            [
+                { "h": "i" },
+                { "d": "j" },
+            ],
+            ["k", "l"],
+          ]
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a": ["b", "c", "f", "g", "k", "l"],
+                "a.d": ["e", "j"],
+                "a.h": "i",
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+
+    #[test]
+    fn flatten_nested_arrays_and_objects() {
+        let mut base: Value = json!({
+          "a": [
+            "b",
+            ["c", "d"],
+            { "e": ["f", "g"] },
+            [
+                { "h": "i" },
+                { "e": ["j", { "z": "y" }] },
+            ],
+            ["l"],
+            "m",
+          ]
+        });
+        let json = std::mem::take(base.as_object_mut().unwrap());
+        let flat = flatten(&json);
+
+        println!("{}", serde_json::to_string_pretty(&flat).unwrap());
+
+        assert_eq!(
+            &flat,
+            json!({
+                "a": ["b", "c", "d", "l", "m"],
+                "a.e": ["f", "g", "j"],
+                "a.h": "i",
+                "a.e.z": "y",
+            })
+            .as_object()
+            .unwrap()
+        );
+    }
+}
diff --git a/flatten-serde-json/src/main.rs b/flatten-serde-json/src/main.rs
new file mode 100644
index 000000000..dabb386f1
--- /dev/null
+++ b/flatten-serde-json/src/main.rs
@@ -0,0 +1,11 @@
+use std::io::stdin;
+
+use flatten_serde_json::flatten;
+use serde_json::{Map, Value};
+
+fn main() {
+    let json: Map<String, Value> = serde_json::from_reader(stdin()).unwrap();
+
+    let result = flatten(&json);
+    println!("{}", serde_json::to_string_pretty(&result).unwrap());
+}
diff --git a/milli/Cargo.toml b/milli/Cargo.toml
index e8723dc6a..a83cfd6f2 100644
--- a/milli/Cargo.toml
+++ b/milli/Cargo.toml
@@ -14,7 +14,7 @@ crossbeam-channel = "0.5.2"
 either = "1.6.1"
 fst = "0.4.7"
 fxhash = "0.2.1"
-flatten-serde-json = "0.1.0"
+flatten-serde-json = { path = "../flatten-serde-json" }
 grenad = { version = "0.4.1", default-features = false, features = ["tempfile"] }
 geoutils = "0.4.1"
 heed = { git = "https://github.com/meilisearch/heed", tag = "v0.12.1", default-features = false, features = ["lmdb", "sync-read-txn"] }