diff --git a/milli/src/documents/builder.rs b/milli/src/documents/builder.rs
index 391175f31..159afb8d9 100644
--- a/milli/src/documents/builder.rs
+++ b/milli/src/documents/builder.rs
@@ -1,157 +1,159 @@
-use std::collections::BTreeMap;
-use std::io;
-use std::io::{Cursor, Write};
+use std::io::{self, Write};
 
-use byteorder::{BigEndian, WriteBytesExt};
-use serde::Deserializer;
-use serde_json::Value;
+use grenad::{CompressionType, WriterBuilder};
+use serde_json::{to_writer, Map, Value};
 
-use super::serde_impl::DocumentVisitor;
-use super::{ByteCounter, DocumentsBatchIndex, DocumentsMetadata, Error};
-use crate::FieldId;
+use super::{DocumentsBatchIndex, Error, DOCUMENTS_BATCH_INDEX_KEY};
 
 /// The `DocumentsBatchBuilder` provides a way to build a documents batch in the intermediary
 /// format used by milli.
 ///
-/// The writer used by the DocumentBatchBuilder can be read using a `DocumentBatchReader` to
-/// iterate over the documents.
+/// The writer used by the `DocumentsBatchBuilder` can be read using a `DocumentsBatchReader`
+/// to iterate over the documents.
 ///
 /// ## example:
 /// ```
-/// use milli::documents::DocumentBatchBuilder;
 /// use serde_json::json;
-/// use std::io::Cursor;
+/// use milli::documents::DocumentsBatchBuilder;
 ///
-/// let json = r##"{"id": 1, "name": "foo"}"##;
-/// let mut writer = Cursor::new(Vec::new());
-/// let mut builder = DocumentBatchBuilder::new(&mut writer).unwrap();
-/// builder.extend_from_json(&mut json.as_bytes()).unwrap();
-/// builder.finish().unwrap();
+/// let json = json!({ "id": 1, "name": "foo" });
+///
+/// let mut builder = DocumentsBatchBuilder::new(Vec::new());
+/// builder.append_json_object(json.as_object().unwrap()).unwrap();
+/// let _vector = builder.into_inner().unwrap();
 /// ```
-pub struct DocumentBatchBuilder<W> {
-    inner: ByteCounter<W>,
-    index: DocumentsBatchIndex,
+pub struct DocumentsBatchBuilder<W> {
+    /// The inner grenad writer, the last value must always be the `DocumentsBatchIndex`.
+    writer: grenad::Writer<W>,
+    /// A map that creates the relation between field ids and field names.
+    fields_index: DocumentsBatchIndex,
+    /// The number of documents that were added to this builder,
+    /// it doesn't take the primary key of the documents into account at this point.
+    documents_count: u32,
+
+    /// A buffer to store a temporary obkv buffer and avoid reallocating.
     obkv_buffer: Vec<u8>,
+    /// A buffer to serialize the values and avoid reallocating,
+    /// serialized values are stored in an obkv.
     value_buffer: Vec<u8>,
-    values: BTreeMap<FieldId, Value>,
-    count: usize,
 }
 
-impl<W: io::Write + io::Seek> DocumentBatchBuilder<W> {
-    pub fn new(writer: W) -> Result<Self, Error> {
-        let index = DocumentsBatchIndex::default();
-        let mut writer = ByteCounter::new(writer);
-        // add space to write the offset of the metadata at the end of the writer
-        writer.write_u64::<BigEndian>(0)?;
-
-        Ok(Self {
-            inner: writer,
-            index,
+impl<W: Write> DocumentsBatchBuilder<W> {
+    pub fn new(writer: W) -> DocumentsBatchBuilder<W> {
+        DocumentsBatchBuilder {
+            writer: WriterBuilder::new().compression_type(CompressionType::None).build(writer),
+            fields_index: DocumentsBatchIndex::default(),
+            documents_count: 0,
             obkv_buffer: Vec::new(),
             value_buffer: Vec::new(),
-            values: BTreeMap::new(),
-            count: 0,
-        })
+        }
     }
 
-    /// Returns the number of documents that have been written to the builder.
-    pub fn len(&self) -> usize {
-        self.count
+    /// Returns the number of documents inserted into this builder.
+    pub fn documents_count(&self) -> u32 {
+        self.documents_count
     }
 
-    /// This method must be called after the document addition is terminated. It will put the
-    /// metadata at the end of the file, and write the metadata offset at the beginning on the
-    /// file.
-    pub fn finish(self) -> Result<usize, Error> {
-        let Self { inner: ByteCounter { mut writer, count: offset }, index, count, .. } = self;
+    /// Appends a new JSON object into the batch and updates the `DocumentsBatchIndex` accordingly.
+    pub fn append_json_object(&mut self, object: &Map<String, Value>) -> io::Result<()> {
+        // Make sure that we insert the fields ids in order as the obkv writer has this requirement.
+        let mut fields_ids: Vec<_> = object.keys().map(|k| self.fields_index.insert(&k)).collect();
+        fields_ids.sort_unstable();
 
-        let meta = DocumentsMetadata { count, index };
+        self.obkv_buffer.clear();
+        let mut writer = obkv::KvWriter::new(&mut self.obkv_buffer);
+        for field_id in fields_ids {
+            let key = self.fields_index.name(field_id).unwrap();
+            self.value_buffer.clear();
+            to_writer(&mut self.value_buffer, &object[key])?;
+            writer.insert(field_id, &self.value_buffer)?;
+        }
 
-        bincode::serialize_into(&mut writer, &meta)?;
+        let internal_id = self.documents_count.to_be_bytes();
+        let document_bytes = writer.into_inner()?;
+        self.writer.insert(internal_id, &document_bytes)?;
+        self.documents_count += 1;
 
-        writer.seek(io::SeekFrom::Start(0))?;
-        writer.write_u64::<BigEndian>(offset as u64)?;
-
-        writer.flush()?;
-
-        Ok(count)
+        Ok(())
     }
 
-    /// Extends the builder with json documents from a reader.
-    pub fn extend_from_json<R: io::Read>(&mut self, reader: R) -> Result<(), Error> {
-        let mut de = serde_json::Deserializer::from_reader(reader);
-
-        let mut visitor = DocumentVisitor {
-            inner: &mut self.inner,
-            index: &mut self.index,
-            obkv_buffer: &mut self.obkv_buffer,
-            value_buffer: &mut self.value_buffer,
-            values: &mut self.values,
-            count: &mut self.count,
-        };
-
-        de.deserialize_any(&mut visitor).map_err(Error::JsonError)?
-    }
-
-    /// Creates a builder from a reader of CSV documents.
-    ///
-    /// Since all fields in a csv documents are guaranteed to be ordered, we are able to perform
-    /// optimisations, and extending from another CSV is not allowed.
-    pub fn from_csv<R: io::Read>(reader: R, writer: W) -> Result<Self, Error> {
-        let mut this = Self::new(writer)?;
-        // Ensure that this is the first and only addition made with this builder
-        debug_assert!(this.index.is_empty());
-
-        let mut records = csv::Reader::from_reader(reader);
-
-        let headers = records
+    /// Appends a new CSV file into the batch and updates the `DocumentsBatchIndex` accordingly.
+    pub fn append_csv<R: io::Read>(&mut self, mut reader: csv::Reader<R>) -> Result<(), Error> {
+        // Make sure that we insert the fields ids in order as the obkv writer has this requirement.
+        let mut typed_fields_ids: Vec<_> = reader
             .headers()?
             .into_iter()
             .map(parse_csv_header)
-            .map(|(k, t)| (this.index.insert(k), t))
-            .collect::<BTreeMap<_, _>>();
+            .map(|(k, t)| (self.fields_index.insert(k), t))
+            .enumerate()
+            .collect();
+        typed_fields_ids.sort_unstable_by_key(|(_, (fid, _))| *fid);
 
-        for (i, record) in records.into_records().enumerate() {
-            let record = record?;
-            this.obkv_buffer.clear();
-            let mut writer = obkv::KvWriter::new(&mut this.obkv_buffer);
-            for (value, (fid, ty)) in record.into_iter().zip(headers.iter()) {
-                let value = match ty {
+        let mut record = csv::StringRecord::new();
+        let mut line = 0;
+        while reader.read_record(&mut record)? {
+            // We increment here and not at the end of the while loop to take
+            // the header offset into account.
+            line += 1;
+
+            self.obkv_buffer.clear();
+            let mut writer = obkv::KvWriter::new(&mut self.obkv_buffer);
+
+            for (i, (field_id, type_)) in typed_fields_ids.iter() {
+                self.value_buffer.clear();
+
+                let value = &record[*i];
+                match type_ {
                     AllowedType::Number => {
                         if value.trim().is_empty() {
-                            Value::Null
+                            to_writer(&mut self.value_buffer, &Value::Null)?;
                         } else {
-                            value.trim().parse::<f64>().map(Value::from).map_err(|error| {
-                                Error::ParseFloat {
-                                    error,
-                                    // +1 for the header offset.
-                                    line: i + 1,
-                                    value: value.to_string(),
+                            match value.trim().parse::<f64>() {
+                                Ok(float) => {
+                                    to_writer(&mut self.value_buffer, &float)?;
                                 }
-                            })?
+                                Err(error) => {
+                                    return Err(Error::ParseFloat {
+                                        error,
+                                        line,
+                                        value: value.to_string(),
+                                    });
+                                }
+                            }
                         }
                     }
                     AllowedType::String => {
                         if value.is_empty() {
-                            Value::Null
+                            to_writer(&mut self.value_buffer, &Value::Null)?;
                         } else {
-                            Value::String(value.to_string())
+                            to_writer(&mut self.value_buffer, value)?;
                         }
                     }
-                };
+                }
 
-                this.value_buffer.clear();
-                serde_json::to_writer(Cursor::new(&mut this.value_buffer), &value)?;
-                writer.insert(*fid, &this.value_buffer)?;
+                // We insert into the obkv writer the value buffer that has been filled just above.
+                writer.insert(*field_id, &self.value_buffer)?;
             }
 
-            this.inner.write_u32::<BigEndian>(this.obkv_buffer.len() as u32)?;
-            this.inner.write_all(&this.obkv_buffer)?;
-
-            this.count += 1;
+            let internal_id = self.documents_count.to_be_bytes();
+            let document_bytes = writer.into_inner()?;
+            self.writer.insert(internal_id, &document_bytes)?;
+            self.documents_count += 1;
         }
 
-        Ok(this)
+        Ok(())
+    }
+
+    /// Flushes the content on disk and stores the final version of the `DocumentsBatchIndex`.
+    pub fn into_inner(mut self) -> io::Result<W> {
+        let DocumentsBatchBuilder { mut writer, fields_index, .. } = self;
+
+        // We serialize and insert the `DocumentsBatchIndex` as the last key of the grenad writer.
+        self.value_buffer.clear();
+        to_writer(&mut self.value_buffer, &fields_index)?;
+        writer.insert(DOCUMENTS_BATCH_INDEX_KEY, &self.value_buffer)?;
+
+        writer.into_inner()
     }
 }
 
diff --git a/milli/src/documents/mod.rs b/milli/src/documents/mod.rs
index 09f15901d..bd0afc6e4 100644
--- a/milli/src/documents/mod.rs
+++ b/milli/src/documents/mod.rs
@@ -1,24 +1,22 @@
 mod builder;
-/// The documents module defines an intermediary document format that milli uses for indexation, and
-/// provides an API to easily build and read such documents.
-///
-/// The `DocumentBatchBuilder` interface allows to write batches of documents to a writer, that can
-/// later be read by milli using the `DocumentBatchReader` interface.
 mod reader;
-mod serde_impl;
 
 use std::fmt::{self, Debug};
 use std::io;
 
 use bimap::BiHashMap;
-pub use builder::DocumentBatchBuilder;
-pub use reader::DocumentBatchReader;
+pub use builder::DocumentsBatchBuilder;
+pub use reader::{DocumentsBatchCursor, DocumentsBatchReader};
 use serde::{Deserialize, Serialize};
 
 use crate::FieldId;
 
+/// The key that is used to store the `DocumentsBatchIndex` datastructure,
+/// it is the absolute last key of the list.
+const DOCUMENTS_BATCH_INDEX_KEY: [u8; 8] = u64::MAX.to_be_bytes();
+
 /// A bidirectional map that links field ids to their name in a document batch.
-#[derive(Default, Debug, Serialize, Deserialize)]
+#[derive(Default, Clone, Debug, Serialize, Deserialize)]
 pub struct DocumentsBatchIndex(pub BiHashMap<FieldId, String>);
 
 impl DocumentsBatchIndex {
@@ -46,8 +44,8 @@ impl DocumentsBatchIndex {
         self.0.iter()
     }
 
-    pub fn name(&self, id: FieldId) -> Option<&String> {
-        self.0.get_by_left(&id)
+    pub fn name(&self, id: FieldId) -> Option<&str> {
+        self.0.get_by_left(&id).map(AsRef::as_ref)
     }
 
     pub fn recreate_json(
@@ -69,50 +67,20 @@ impl DocumentsBatchIndex {
     }
 }
 
-#[derive(Debug, Serialize, Deserialize)]
-struct DocumentsMetadata {
-    count: usize,
-    index: DocumentsBatchIndex,
-}
-
-pub struct ByteCounter<W> {
-    count: usize,
-    writer: W,
-}
-
-impl<W> ByteCounter<W> {
-    fn new(writer: W) -> Self {
-        Self { count: 0, writer }
-    }
-}
-
-impl<W: io::Write> io::Write for ByteCounter<W> {
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        let count = self.writer.write(buf)?;
-        self.count += count;
-        Ok(count)
-    }
-
-    fn flush(&mut self) -> io::Result<()> {
-        self.writer.flush()
-    }
-}
-
 #[derive(Debug)]
 pub enum Error {
     ParseFloat { error: std::num::ParseFloatError, line: usize, value: String },
     InvalidDocumentFormat,
-    Custom(String),
-    JsonError(serde_json::Error),
-    CsvError(csv::Error),
-    Serialize(bincode::Error),
+    Csv(csv::Error),
+    Json(serde_json::Error),
+    Serialize(serde_json::Error),
+    Grenad(grenad::Error),
     Io(io::Error),
-    DocumentTooLarge,
 }
 
 impl From<csv::Error> for Error {
     fn from(e: csv::Error) -> Self {
-        Self::CsvError(e)
+        Self::Csv(e)
     }
 }
 
@@ -122,15 +90,15 @@ impl From<io::Error> for Error {
     }
 }
 
-impl From<bincode::Error> for Error {
-    fn from(other: bincode::Error) -> Self {
-        Self::Serialize(other)
+impl From<serde_json::Error> for Error {
+    fn from(other: serde_json::Error) -> Self {
+        Self::Json(other)
     }
 }
 
-impl From<serde_json::Error> for Error {
-    fn from(other: serde_json::Error) -> Self {
-        Self::JsonError(other)
+impl From<grenad::Error> for Error {
+    fn from(other: grenad::Error) -> Self {
+        Self::Grenad(other)
     }
 }
 
@@ -140,13 +108,14 @@ impl fmt::Display for Error {
             Error::ParseFloat { error, line, value } => {
                 write!(f, "Error parsing number {:?} at line {}: {}", value, line, error)
             }
-            Error::Custom(s) => write!(f, "Unexpected serialization error: {}", s),
-            Error::InvalidDocumentFormat => f.write_str("Invalid document addition format."),
-            Error::JsonError(err) => write!(f, "Couldn't serialize document value: {}", err),
+            Error::InvalidDocumentFormat => {
+                f.write_str("Invalid document addition format, missing the documents batch index.")
+            }
             Error::Io(e) => write!(f, "{}", e),
-            Error::DocumentTooLarge => f.write_str("Provided document is too large (>2Gib)"),
             Error::Serialize(e) => write!(f, "{}", e),
-            Error::CsvError(e) => write!(f, "{}", e),
+            Error::Grenad(e) => write!(f, "{}", e),
+            Error::Csv(e) => write!(f, "{}", e),
+            Error::Json(e) => write!(f, "{}", e),
         }
     }
 }
@@ -158,15 +127,25 @@ impl std::error::Error for Error {}
 macro_rules! documents {
     ($data:tt) => {{
         let documents = serde_json::json!($data);
-        let mut writer = std::io::Cursor::new(Vec::new());
-        let mut builder = crate::documents::DocumentBatchBuilder::new(&mut writer).unwrap();
-        let documents = serde_json::to_vec(&documents).unwrap();
-        builder.extend_from_json(std::io::Cursor::new(documents)).unwrap();
-        builder.finish().unwrap();
+        let documents = match documents {
+            object @ serde_json::Value::Object(_) => vec![object],
+            serde_json::Value::Array(objects) => objects,
+            invalid => {
+                panic!("an array of objects must be specified, {:#?} is not an array", invalid)
+            }
+        };
 
-        writer.set_position(0);
+        let mut builder = crate::documents::DocumentsBatchBuilder::new(Vec::new());
+        for document in documents {
+            let object = match document {
+                serde_json::Value::Object(object) => object,
+                invalid => panic!("an object must be specified, {:#?} is not an object", invalid),
+            };
+            builder.append_json_object(&object).unwrap();
+        }
 
-        crate::documents::DocumentBatchReader::from_reader(writer).unwrap()
+        let vector = builder.into_inner().unwrap();
+        crate::documents::DocumentsBatchReader::from_reader(std::io::Cursor::new(vector)).unwrap()
     }};
 }
 
diff --git a/milli/src/documents/reader.rs b/milli/src/documents/reader.rs
index 14d7c8ceb..3dff999f5 100644
--- a/milli/src/documents/reader.rs
+++ b/milli/src/documents/reader.rs
@@ -1,11 +1,9 @@
+use std::convert::TryInto;
 use std::io;
-use std::io::{BufReader, Read};
-use std::mem::size_of;
 
-use byteorder::{BigEndian, ReadBytesExt};
 use obkv::KvReader;
 
-use super::{DocumentsBatchIndex, DocumentsMetadata, Error};
+use super::{DocumentsBatchIndex, Error, DOCUMENTS_BATCH_INDEX_KEY};
 use crate::FieldId;
 
 /// The `DocumentsBatchReader` provides a way to iterate over documents that have been created with
@@ -13,63 +11,80 @@ use crate::FieldId;
 ///
 /// The documents are returned in the form of `obkv::Reader` where each field is identified with a
 /// `FieldId`. The mapping between the field ids and the field names is done thanks to the index.
-pub struct DocumentBatchReader<R> {
-    reader: BufReader<R>,
-    metadata: DocumentsMetadata,
-    buffer: Vec<u8>,
-    seen_documents: usize,
+pub struct DocumentsBatchReader<R> {
+    cursor: grenad::ReaderCursor<R>,
+    fields_index: DocumentsBatchIndex,
 }
 
-impl<R: io::Read + io::Seek> DocumentBatchReader<R> {
+impl<R: io::Read + io::Seek> DocumentsBatchReader<R> {
     /// Construct a `DocumentsReader` from a reader.
     ///
-    /// It first retrieves the index, then moves to the first document. Subsequent calls to
-    /// `next_document` advance the document reader until all the documents have been read.
-    pub fn from_reader(mut reader: R) -> Result<Self, Error> {
-        let mut buffer = Vec::new();
+    /// It first retrieves the index, then moves to the first document. Use the `into_cursor`
+    /// method to iterator over the documents, from the first to the last.
+    pub fn from_reader(reader: R) -> Result<Self, Error> {
+        let reader = grenad::Reader::new(reader)?;
+        let mut cursor = reader.into_cursor()?;
 
-        let meta_offset = reader.read_u64::<BigEndian>()?;
-        reader.seek(io::SeekFrom::Start(meta_offset))?;
-        reader.read_to_end(&mut buffer)?;
-        let metadata: DocumentsMetadata = bincode::deserialize(&buffer)?;
+        let fields_index = match cursor.move_on_key_equal_to(DOCUMENTS_BATCH_INDEX_KEY)? {
+            Some((_, value)) => serde_json::from_slice(value).map_err(Error::Serialize)?,
+            None => return Err(Error::InvalidDocumentFormat),
+        };
 
-        reader.seek(io::SeekFrom::Start(size_of::<u64>() as u64))?;
-        buffer.clear();
-
-        let reader = BufReader::new(reader);
-
-        Ok(Self { reader, metadata, buffer, seen_documents: 0 })
+        Ok(DocumentsBatchReader { cursor, fields_index })
     }
 
-    /// Returns the next document in the reader, and wraps it in an `obkv::KvReader`, along with a
-    /// reference to the addition index.
-    pub fn next_document_with_index<'a>(
-        &'a mut self,
-    ) -> io::Result<Option<(&'a DocumentsBatchIndex, KvReader<'a, FieldId>)>> {
-        if self.seen_documents < self.metadata.count {
-            let doc_len = self.reader.read_u32::<BigEndian>()?;
-            self.buffer.resize(doc_len as usize, 0);
-            self.reader.read_exact(&mut self.buffer)?;
-            self.seen_documents += 1;
-
-            let reader = KvReader::new(&self.buffer);
-            Ok(Some((&self.metadata.index, reader)))
-        } else {
-            Ok(None)
-        }
-    }
-
-    /// Return the fields index for the documents batch.
-    pub fn index(&self) -> &DocumentsBatchIndex {
-        &self.metadata.index
-    }
-
-    /// Returns the number of documents in the reader.
-    pub fn len(&self) -> usize {
-        self.metadata.count
+    pub fn documents_count(&self) -> u32 {
+        self.cursor.len().saturating_sub(1).try_into().expect("Invalid number of documents")
     }
 
     pub fn is_empty(&self) -> bool {
-        self.len() == 0
+        self.cursor.len().saturating_sub(1) == 0
+    }
+
+    pub fn documents_batch_index(&self) -> &DocumentsBatchIndex {
+        &self.fields_index
+    }
+
+    /// This method returns a forward cursor over the documents.
+    pub fn into_cursor(self) -> DocumentsBatchCursor<R> {
+        let DocumentsBatchReader { cursor, fields_index } = self;
+        let mut cursor = DocumentsBatchCursor { cursor, fields_index };
+        cursor.reset();
+        cursor
+    }
+}
+
+/// A forward cursor over the documents in a `DocumentsBatchReader`.
+pub struct DocumentsBatchCursor<R> {
+    cursor: grenad::ReaderCursor<R>,
+    fields_index: DocumentsBatchIndex,
+}
+
+impl<R> DocumentsBatchCursor<R> {
+    pub fn into_reader(self) -> DocumentsBatchReader<R> {
+        let DocumentsBatchCursor { cursor, fields_index, .. } = self;
+        DocumentsBatchReader { cursor, fields_index }
+    }
+
+    pub fn documents_batch_index(&self) -> &DocumentsBatchIndex {
+        &self.fields_index
+    }
+
+    /// Resets the cursor to be able to read from the start again.
+    pub fn reset(&mut self) {
+        self.cursor.reset();
+    }
+}
+
+impl<R: io::Read + io::Seek> DocumentsBatchCursor<R> {
+    /// Returns the next document, starting from the first one. Subsequent calls to
+    /// `next_document` advance the document reader until all the documents have been read.
+    pub fn next_document(&mut self) -> Result<Option<KvReader<FieldId>>, grenad::Error> {
+        match self.cursor.move_on_next()? {
+            Some((key, value)) if key != DOCUMENTS_BATCH_INDEX_KEY => {
+                Ok(Some(KvReader::new(value)))
+            }
+            _otherwise => Ok(None),
+        }
     }
 }
diff --git a/milli/src/documents/serde_impl.rs b/milli/src/documents/serde_impl.rs
deleted file mode 100644
index d57bf1ffb..000000000
--- a/milli/src/documents/serde_impl.rs
+++ /dev/null
@@ -1,134 +0,0 @@
-use std::collections::BTreeMap;
-use std::fmt;
-use std::io::{Cursor, Write};
-
-use byteorder::WriteBytesExt;
-use serde::de::{DeserializeSeed, MapAccess, SeqAccess, Visitor};
-use serde::Deserialize;
-use serde_json::Value;
-
-use super::{ByteCounter, DocumentsBatchIndex, Error};
-use crate::FieldId;
-
-macro_rules! tri {
-    ($e:expr) => {
-        match $e {
-            Ok(r) => r,
-            Err(e) => return Ok(Err(e.into())),
-        }
-    };
-}
-
-struct FieldIdResolver<'a>(&'a mut DocumentsBatchIndex);
-
-impl<'a, 'de> DeserializeSeed<'de> for FieldIdResolver<'a> {
-    type Value = FieldId;
-
-    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
-    where
-        D: serde::Deserializer<'de>,
-    {
-        deserializer.deserialize_str(self)
-    }
-}
-
-impl<'a, 'de> Visitor<'de> for FieldIdResolver<'a> {
-    type Value = FieldId;
-
-    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
-    where
-        E: serde::de::Error,
-    {
-        Ok(self.0.insert(v))
-    }
-
-    fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "a string")
-    }
-}
-
-struct ValueDeserializer;
-
-impl<'de> DeserializeSeed<'de> for ValueDeserializer {
-    type Value = serde_json::Value;
-
-    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
-    where
-        D: serde::Deserializer<'de>,
-    {
-        serde_json::Value::deserialize(deserializer)
-    }
-}
-
-pub struct DocumentVisitor<'a, W> {
-    pub inner: &'a mut ByteCounter<W>,
-    pub index: &'a mut DocumentsBatchIndex,
-    pub obkv_buffer: &'a mut Vec<u8>,
-    pub value_buffer: &'a mut Vec<u8>,
-    pub values: &'a mut BTreeMap<FieldId, Value>,
-    pub count: &'a mut usize,
-}
-
-impl<'a, 'de, W: Write> Visitor<'de> for &mut DocumentVisitor<'a, W> {
-    /// This Visitor value is nothing, since it write the value to a file.
-    type Value = Result<(), Error>;
-
-    fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
-    where
-        A: SeqAccess<'de>,
-    {
-        while let Some(v) = seq.next_element_seed(&mut *self)? {
-            tri!(v)
-        }
-
-        Ok(Ok(()))
-    }
-
-    fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
-    where
-        A: MapAccess<'de>,
-    {
-        while let Some((key, value)) =
-            map.next_entry_seed(FieldIdResolver(&mut *self.index), ValueDeserializer)?
-        {
-            self.values.insert(key, value);
-        }
-
-        self.obkv_buffer.clear();
-        let mut obkv = obkv::KvWriter::new(Cursor::new(&mut *self.obkv_buffer));
-        for (key, value) in self.values.iter() {
-            self.value_buffer.clear();
-            // This is guaranteed to work
-            tri!(serde_json::to_writer(Cursor::new(&mut *self.value_buffer), value));
-            tri!(obkv.insert(*key, &self.value_buffer));
-        }
-
-        let reader = tri!(obkv.into_inner()).into_inner();
-
-        tri!(self.inner.write_u32::<byteorder::BigEndian>(reader.len() as u32));
-        tri!(self.inner.write_all(reader));
-
-        *self.count += 1;
-        self.values.clear();
-
-        Ok(Ok(()))
-    }
-
-    fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "a documents, or a sequence of documents.")
-    }
-}
-
-impl<'a, 'de, W> DeserializeSeed<'de> for &mut DocumentVisitor<'a, W>
-where
-    W: Write,
-{
-    type Value = Result<(), Error>;
-
-    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
-    where
-        D: serde::Deserializer<'de>,
-    {
-        deserializer.deserialize_map(self)
-    }
-}