Deserialize series using field names, not order (#62)

SafariMonkey · web-flow · commit 2d648f3712e3 · 2020-09-22T23:02:06.000+02:00
Add a custom Deserialize impl for Series which uses the field names
in the columns field of the response to allow the inner type
to deserialize itself as a map instead of relying on the field order.
diff --git a/influxdb/src/integrations/serde_integration/de.rs b/influxdb/src/integrations/serde_integration/de.rs
@@ -0,0 +1,346 @@
+use super::Series;
+use serde::de::{
+    value, DeserializeSeed, Deserializer, Error, IntoDeserializer, MapAccess, SeqAccess, Visitor,
+};
+use serde::Deserialize;
+use std::fmt;
+use std::marker::PhantomData;
+
+// Based on https://serde.rs/deserialize-struct.html
+impl<'de, T> Deserialize<'de> for Series<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // Field name deserializer
+        #[derive(Deserialize)]
+        #[serde(field_identifier, rename_all = "lowercase")]
+        enum Field {
+            Name,
+            Columns,
+            Values,
+        };
+
+        struct SeriesVisitor<T> {
+            _inner_type: PhantomData<T>,
+        };
+
+        impl<'de, T> Visitor<'de> for SeriesVisitor<T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = Series<T>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("struct Series")
+            }
+
+            fn visit_map<V>(self, mut map: V) -> Result<Series<T>, V::Error>
+            where
+                V: MapAccess<'de>,
+            {
+                let mut name = None;
+                let mut columns: Option<Vec<String>> = None;
+                let mut values: Option<Vec<T>> = None;
+                while let Some(key) = map.next_key()? {
+                    match key {
+                        Field::Name => {
+                            if name.is_some() {
+                                return Err(Error::duplicate_field("name"));
+                            }
+                            name = Some(map.next_value()?);
+                        }
+                        Field::Columns => {
+                            if columns.is_some() {
+                                return Err(Error::duplicate_field("columns"));
+                            }
+                            columns = Some(map.next_value()?);
+                        }
+                        Field::Values => {
+                            if values.is_some() {
+                                return Err(Error::duplicate_field("values"));
+                            }
+                            // Error out if "values" is encountered before "columns"
+                            // Hopefully, InfluxDB never does this.
+                            if columns.is_none() {
+                                return Err(Error::custom(
+                                    "series values encountered before columns",
+                                ));
+                            }
+                            // Deserialize using a HeaderVec deserializer
+                            // seeded with the headers from the "columns" field
+                            values = Some(map.next_value_seed(HeaderVec::<T> {
+                                header: columns.as_ref().unwrap(),
+                                _inner_type: PhantomData,
+                            })?);
+                        }
+                    }
+                }
+                let name = name.ok_or_else(|| Error::missing_field("name"))?;
+                let values = values.ok_or_else(|| Error::missing_field("values"))?;
+                Ok(Series { name, values })
+            }
+        }
+
+        const FIELDS: &[&str] = &["name", "values"];
+        deserializer.deserialize_struct(
+            "Series",
+            FIELDS,
+            SeriesVisitor::<T> {
+                _inner_type: PhantomData,
+            },
+        )
+    }
+}
+
+// Deserializer that takes a header as a seed
+// and deserializes an array of arrays into a
+// Vec of map-like values using the header as
+// keys and the values as values.
+struct HeaderVec<'h, T> {
+    header: &'h [String],
+    _inner_type: PhantomData<T>,
+}
+
+impl<'de, 'h, T> DeserializeSeed<'de> for HeaderVec<'h, T>
+where
+    T: Deserialize<'de>,
+{
+    type Value = Vec<T>;
+
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct HeaderVecVisitor<'h, T> {
+            header: &'h [String],
+            _inner_type: PhantomData<T>,
+        }
+        impl<'de, 'h, T> Visitor<'de> for HeaderVecVisitor<'h, T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = Vec<T>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                write!(formatter, "an array of arrays")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                let mut vec = Vec::new();
+
+                while let Some(v) = seq.next_element_seed(RowWithHeader {
+                    header: self.header,
+                    _inner_type: PhantomData,
+                })? {
+                    vec.push(v);
+                }
+
+                Ok(vec)
+            }
+        }
+        deserializer.deserialize_seq(HeaderVecVisitor {
+            header: self.header,
+            _inner_type: PhantomData,
+        })
+    }
+}
+
+// Deserializer that takes a header as a seed
+// and deserializes an array into a map-like
+// value using the header as keys and the values
+// as values.
+struct RowWithHeader<'h, T> {
+    header: &'h [String],
+    _inner_type: PhantomData<T>,
+}
+
+impl<'de, 'h, T> DeserializeSeed<'de> for RowWithHeader<'h, T>
+where
+    T: Deserialize<'de>,
+{
+    type Value = T;
+
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct RowWithHeaderVisitor<'h, T> {
+            header: &'h [String],
+            _inner: PhantomData<fn() -> T>,
+        }
+
+        impl<'de, 'h, T> Visitor<'de> for RowWithHeaderVisitor<'h, T>
+        where
+            T: Deserialize<'de>,
+        {
+            type Value = T;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("array")
+            }
+
+            fn visit_seq<A>(self, seq: A) -> Result<T, A::Error>
+            where
+                A: SeqAccess<'de>,
+            {
+                // `MapAccessDeserializer` is a wrapper that turns a `MapAccess`
+                // into a `Deserializer`, allowing it to be used as the input to T's
+                // `Deserialize` implementation. T then deserializes itself using
+                // the entries from the map visitor.
+                Deserialize::deserialize(value::MapAccessDeserializer::new(HeaderMapAccess {
+                    header: self.header,
+                    field: 0,
+                    data: seq,
+                }))
+            }
+        }
+
+        deserializer.deserialize_seq(RowWithHeaderVisitor {
+            header: self.header,
+            _inner: PhantomData,
+        })
+    }
+}
+
+// MapAccess implementation that holds a reference to
+// the header for keys and a serde sequence for values.
+// When asked for a key, it returns the next header and
+// advances its header field index. When asked for a value,
+// it tries to deserialize the next element in the serde
+// sequence into the desired type, and returns an error
+// if no element is returned (the sequence is exhausted).
+struct HeaderMapAccess<'h, A> {
+    header: &'h [String],
+    field: usize,
+    data: A,
+}
+
+impl<'de, 'h, A> MapAccess<'de> for HeaderMapAccess<'h, A>
+where
+    A: SeqAccess<'de>,
+{
+    type Error = <A as SeqAccess<'de>>::Error;
+
+    fn next_key_seed<K: DeserializeSeed<'de>>(
+        &mut self,
+        seed: K,
+    ) -> Result<Option<K::Value>, Self::Error> {
+        let field = match self.header.get(self.field) {
+            None => return Ok(None),
+            Some(field) => field,
+        };
+        self.field += 1;
+        seed.deserialize(field.clone().into_deserializer())
+            .map(Some)
+    }
+
+    fn next_value_seed<K: DeserializeSeed<'de>>(
+        &mut self,
+        seed: K,
+    ) -> Result<K::Value, Self::Error> {
+        match self.data.next_element_seed(seed)? {
+            Some(value) => Ok(value),
+            None => Err(Error::custom("next_value_seed called but no value")),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Series;
+    use std::borrow::Cow;
+    use std::collections::HashMap;
+
+    const TEST_DATA: &str = r#"
+    {
+        "name": "series_name",
+        "columns": ["foo", "bar"],
+        "values": [
+            ["foo_a", "bar_a"],
+            ["foo_b", "bar_b"]
+        ]
+    }
+    "#;
+
+    // we can derive all the impls we want here
+    #[derive(Debug, PartialEq, Eq)]
+    struct EqSeries<T> {
+        pub name: String,
+        pub values: Vec<T>,
+    }
+
+    impl<T> From<Series<T>> for EqSeries<T> {
+        fn from(Series { name, values }: Series<T>) -> Self {
+            EqSeries { name, values }
+        }
+    }
+
+    #[test]
+    fn test_deserialize_cow() {
+        // Unfortunately, Cow is not automatically borrowed,
+        // so this is basically equivalent to String, String
+        let result = serde_json::from_str::<Series<HashMap<Cow<str>, Cow<str>>>>(TEST_DATA);
+        assert!(result.is_ok());
+        assert_eq!(
+            EqSeries::from(result.unwrap()),
+            EqSeries {
+                name: "series_name".into(),
+                values: vec![
+                    {
+                        let mut h = std::collections::HashMap::new();
+                        h.insert("foo".into(), "foo_a".into());
+                        h.insert("bar".into(), "bar_a".into());
+                        h
+                    },
+                    {
+                        let mut h = std::collections::HashMap::new();
+                        h.insert("foo".into(), "foo_b".into());
+                        h.insert("bar".into(), "bar_b".into());
+                        h
+                    },
+                ],
+            },
+        );
+    }
+
+    #[test]
+    fn test_deserialize_borrowed() {
+        use serde::Deserialize;
+
+        // Deserializing a string that cannot be passed through
+        // without escaping will result in an error like this:
+        // `invalid type: string "\n", expected a borrowed string at line 6 column 43`
+        // but if it doesn't need escaping it's fine.
+        #[derive(Deserialize, Debug, PartialEq, Eq)]
+        struct BorrowingStruct<'a> {
+            foo: &'a str,
+            bar: &'a str,
+        }
+
+        let result = serde_json::from_str::<Series<BorrowingStruct>>(TEST_DATA);
+        assert!(result.is_ok(), "{}", result.unwrap_err());
+        assert_eq!(
+            EqSeries::from(result.unwrap()),
+            EqSeries {
+                name: "series_name".into(),
+                values: vec![
+                    BorrowingStruct {
+                        foo: "foo_a",
+                        bar: "bar_a",
+                    },
+                    BorrowingStruct {
+                        foo: "foo_b",
+                        bar: "bar_b",
+                    },
+                ],
+            },
+        );
+    }
+}
diff --git a/influxdb/src/integrations/serde_integration/mod.rs b/influxdb/src/integrations/serde_integration/mod.rs
@@ -46,6 +46,8 @@
 //! # }
 //! ```
 
+mod de;
+
 use reqwest::{Client as ReqwestClient, StatusCode, Url};
 
 use serde::{de::DeserializeOwned, Deserialize};
@@ -84,7 +86,7 @@ pub struct Return<T> {
     pub series: Vec<Series<T>>,
 }
 
-#[derive(Deserialize, Debug)]
+#[derive(Debug)]
 /// Represents a returned series from InfluxDB
 pub struct Series<T> {
     pub name: String,
diff --git a/influxdb/tests/integration_tests.rs b/influxdb/tests/integration_tests.rs
@@ -274,8 +274,10 @@ async fn test_write_and_read_option() {
                 #[derive(Deserialize, Debug, PartialEq)]
                 struct Weather {
                     time: String,
-                    temperature: i32,
+                    // different order to verify field names
+                    // are being used instead of just order
                     wind_strength: Option<u64>,
+                    temperature: i32,
                 }
 
                 let query =
