mapstructure

Overview ¶

Package mapstructure exposes functionality to convert one arbitrary Go type into another, typically to convert a map[string]interface{} into a native Go structure.

The Go structure can be arbitrarily complex, containing slices, other structs, etc. and the decoder will properly decode nested maps and so on into the proper structures in the native Go struct. See the examples to see what the decoder is capable of.

The simplest function to start with is Decode.

Field Tags ¶

When decoding to a struct, mapstructure will use the field name by default to perform the mapping. For example, if a struct has a field "Username" then mapstructure will look for a key in the source value of "username" (case insensitive).

type User struct {
    Username string
}

You can change the behavior of mapstructure by using struct tags. The default struct tag that mapstructure looks for is "mapstructure" but you can customize it using DecoderConfig.

Renaming Fields ¶

To rename the key that mapstructure looks for, use the "mapstructure" tag and set a value directly. For example, to change the "username" example above to "user":

type User struct {
    Username string `mapstructure:"user"`
}

Embedded Structs and Squashing ¶

Embedded structs are treated as if they're another field with that name. By default, the two structs below are equivalent when decoding with mapstructure:

type Person struct {
    Name string
}

type Friend struct {
    Person
}

type Friend struct {
    Person Person
}

This would require an input that looks like below:

map[string]interface{}{
    "person": map[string]interface{}{"name": "alice"},
}

If your "person" value is NOT nested, then you can append ",squash" to your tag value and mapstructure will treat it as if the embedded struct were part of the struct directly. Example:

type Friend struct {
    Person `mapstructure:",squash"`
}

Now the following input would be accepted:

map[string]interface{}{
    "name": "alice",
}

When decoding from a struct to a map, the squash tag squashes the struct fields into a single map. Using the example structs from above:

Friend{Person: Person{Name: "alice"}}

Will be decoded into a map:

map[string]interface{}{
    "name": "alice",
}

DecoderConfig has a field that changes the behavior of mapstructure to always squash embedded structs.

Remainder Values ¶

If there are any unmapped keys in the source value, mapstructure by default will silently ignore them. You can error by setting ErrorUnused in DecoderConfig. If you're using Metadata you can also maintain a slice of the unused keys.

You can also use the ",remain" suffix on your tag to collect all unused values in a map. The field with this tag MUST be a map type and should probably be a "map[string]interface{}" or "map[interface{}]interface{}". See example below:

type Friend struct {
    Name  string
    Other map[string]interface{} `mapstructure:",remain"`
}

Given the input below, Other would be populated with the other values that weren't used (everything but "name"):

map[string]interface{}{
    "name":    "bob",
    "address": "123 Maple St.",
}

Omit Empty Values ¶

When decoding from a struct to any other value, you may use the ",omitempty" suffix on your tag to omit that value if it equates to the zero value. The zero value of all types is specified in the Go specification.

For example, the zero type of a numeric type is zero ("0"). If the struct field value is zero and a numeric type, the field is empty, and it won't be encoded into the destination type.

type Source {
    Age int `mapstructure:",omitempty"`
}

Unexported fields ¶

Since unexported (private) struct fields cannot be set outside the package where they are defined, the decoder will simply skip them.

For this output type definition:

type Exported struct {
    private string // this unexported field will be skipped
    Public string
}

Using this map as input:

map[string]interface{}{
    "private": "I will be ignored",
    "Public":  "I made it through!",
}

The following struct will be decoded:

type Exported struct {
    private: "" // field is left with an empty string (zero value)
    Public: "I made it through!"
}

Other Configuration ¶

mapstructure is highly configurable. See the DecoderConfig struct for other features and options that are supported.