Removing fields from struct or hiding them in JSON Response

Question

I ve created an API in Go that  upon being called  performs a query  creates an instance of a struct  and then encodes that struct as JSON before sending back to the caller   I d now like to allow the caller to be able to select the specific fields they would like returned by passing in a  quot fields quot  GET parameter  This means depending on the fields value s   my struct would change   Is there any way to remove fields from a struct   Or at least hide them in the JSON response dynamically    Note  Sometimes I have empty values so the JSON omitEmpty tag will not work here   If neither of these are possible  is there a suggestion on a better way to handle this  A smaller version of the structs I m using are below  type SearchResult struct       Date        string       json  quot date quot       IdCompany   int          json  quot idCompany quot       Company     string       json  quot company quot       IdIndustry  interface    json  quot idIndustry quot       Industry    string       json  quot industry quot       IdContinent interface    json  quot idContinent quot       Continent   string       json  quot continent quot       IdCountry   interface    json  quot idCountry quot       Country     string       json  quot country quot       IdState     interface    json  quot idState quot       State       string       json  quot state quot       IdCity      interface    json  quot idCity quot       City        string       json  quot city quot       SearchResult  type SearchResults struct       NumberResults int             json  quot numberResults quot       Results         SearchResult  json  quot results quot       type SearchResults  I then encode and output the response like so  err    json NewEncoder c ResponseWriter  Encode  amp msg

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I also faced this problem  at first I just wanted to specialize the responses in my http handler  My first approach was creating a package that copies the information of a struct to another struct and then marshal that second struct  I did that package using reflection  so  never liked that approach and also I wasn t dynamically   So I decided to modify the encoding json package to do this  The functions Marshal  MarshalIndent and  Encoder  Encode additionally receives a  type F map string F   I wanted to simulate a JSON of the fields that are needed to marshal  so it only marshals the fields that are in the map   https   github com JuanTorr jsont  package main  import        fmt       log       net http        github com JuanTorr jsont     type SearchResult struct       Date        string       json  date       IdCompany   int          json  idCompany       Company     string       json  company       IdIndustry  interface    json  idIndustry       Industry    string       json  industry       IdContinent interface    json  idContinent       Continent   string       json  continent       IdCountry   interface    json  idCountry       Country     string       json  country       IdState     interface    json  idState       State       string       json  state       IdCity      interface    json  idCity       City        string       json  city       SearchResult  type SearchResults struct       NumberResults int             json  numberResults       Results         SearchResult  json  results       type SearchResults func main         msg    SearchResults          NumberResults  2          Results    SearchResult                                Date          12-12-12                   IdCompany    1                  Company       alfa                   IdIndustry   1                  Industry      IT                   IdContinent  1                  Continent     america                   IdCountry    1                  Country       M  xico                   IdState      1                  State         CDMX                   IdCity       1                  City          Atz                                                Date          12-12-12                   IdCompany    2                  Company       beta                   IdIndustry   1                  Industry      IT                   IdContinent  1                  Continent     america                   IdCountry    2                  Country       USA                   IdState      2                  State         TX                   IdCity       2                  City          XYZ                                       fmt Println msg      http HandleFunc      func w http ResponseWriter  r  http Request                 numberResults  2  results     date   12-12-12   idCompany  1  idIndustry  1  country   M  xico     date   12-12-12   idCompany  2  idIndustry  1  country   USA             err    jsont NewEncoder w  Encode msg  jsont F               numberResults   nil               results   jsont F                   date         nil                   idCompany    nil                   idIndustry   nil                   country      nil                                    if err    nil               log Fatal err                        http ListenAndServe   3009   nil

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Here is how I defined my structure  type User struct       Username string   json  quot username quot  bson  quot username quot       Email    string   json  quot email quot  bson  quot email quot       Password  string  json  quot password omitempty quot  bson  quot password quot       FullName string   json  quot fullname quot  bson  quot fullname quot      And inside my function set user Password   nil for not to be Marshalled

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The question is now a bit old  but I came across the same issue a little while ago  and as I found no easy way to do this  I built a library fulfilling this purpose  It allows to easily generate a map string interface   from a static struct   https   github com tuvistavie structomap

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Take three ingredients    The reflect package to loop over all the fields of a struct  An if statement to pick up the fields you want to Marshal  and The encoding json package to Marshal the fields of your liking    Preparation    Blend them in a good proportion  Use reflect TypeOf your struct  Field i  Name   to get a name of the ith field of your struct  Use reflect ValueOf your struct  Field i  to get a type Value representation of an ith field of your struct  Use fieldValue Interface   to retrieve the actual value  upcasted to type interface    of the fieldValue of type Value  note the bracket use - the Interface   method produces interface      If you luckily manage not to burn any transistors or circuit-breakers in the process you should get something like this   func MarshalOnlyFields structa interface        includeFields map string bool   jsona   byte  status error        value    reflect ValueOf structa      typa    reflect TypeOf structa      size    value NumField       jsona   append jsona           for i    0  i  lt  size  i             structValue    value Field i          var fieldName string   typa Field i  Name         if marshalledField  marshalStatus    json Marshal  structValue  Interface     marshalStatus    nil               return   byte    marshalStatus           else               if includeFields fieldName                    jsona   append jsona                       jsona   append jsona    byte fieldName                      jsona   append jsona                       jsona   append jsona                       jsona   append jsona   marshalledField                      if i 1    len includeFields                        jsona   append jsona                                                           jsona   append jsona           return     Serving   serve with an arbitrary struct and a map string bool of fields you want to include  for example  type magic struct       Magic1 int     Magic2 string     Magic3  2 int    func main         var magic   magic 0   tusia    2 int 0  1       if json  status    MarshalOnlyFields magic  map string bool  Magic1   true    status    nil           println  error         else           fmt Println string json              Bon Appetit

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I just published sheriff  which transforms structs to a map based on tags annotated on the struct fields  You can then marshal  JSON or others  the generated map  It probably doesn t allow you to only serialize the set of fields the caller requested  but I imagine using a set of groups would allow you to cover most cases  Using groups instead of the fields directly would most likely also increase cache-ability    Example   package main  import        encoding json       fmt       log        github com hashicorp go-version       github com liip sheriff     type User struct       Username string    json  username  groups  api       Email    string    json  email  groups  personal       Name     string    json  name  groups  api       Roles      string  json  roles  groups  api  since  2      func main         user    User          Username   alice           Email      alice example org           Name       Alice           Roles       string  user    admin               v2  err    version NewVersion  2 0 0       if err    nil           log Panic err             o     amp sheriff Options          Groups        string  api            ApiVersion  v2             data  err    sheriff Marshal o  user      if err    nil           log Panic err             output  err    json MarshalIndent data                if err    nil           log Panic err            fmt Printf   s   output

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Other answers already explained that this isn t possible using the encoding json package features directly  Instead  I d like to offer an alternative solution using a third-party library  Disclaimer  I am the author of the library  https   github com wI2L jettison Jettison is a faster and flexible alternative to encoding json  while still retaining a behavior identical to the standard library by default  You can use the DenyList option to filter-out fields that should be omitted from the result  Note that the feature is limited  and only works for 1st-level fields

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Another way to do this is to have a struct of pointers with the  omitempty tag  If the pointers are nil  the fields won t be Marshalled   This method will not require additional reflection or inefficient use of maps   Same example as jorelli using this method  http   play golang org p JJNa0m2 nw

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EDIT  I noticed a few downvotes and took another look at this Q amp A  Most people seem to miss that the OP asked for fields to be dynamically selected based on the caller-provided list of fields  You can t do this with the statically-defined json struct tag   If what you want is to always skip a field to json-encode  then of course use json  -  to ignore the field  also note that this is not required if your field is unexported - those fields are always ignored by the json encoder   But that is not the OP s question   To quote the comment on the json  -  answer      This  the json  -  answer  is the answer most people ending up here from searching would want  but it s not the answer to the question      I d use a map string interface   instead of a struct in this case  You can easily remove fields by calling the delete built-in on the map for the fields to remove   That is  if you can t query only for the requested fields in the first place

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You can use tagging attribute  omitifempty  or make optional fields pointers and leave those you want skipped uninitialized

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use  json  -       Field is ignored by this package  Field int  json  -       Field appears in JSON as key  myName   Field int  json  myName       Field appears in JSON as key  myName  and    the field is omitted from the object if its value is empty     as defined above  Field int  json  myName omitempty       Field appears in JSON as key  Field   the default   but    the field is skipped if empty     Note the leading comma  Field int  json   omitempty     doc   http   golang org pkg encoding json  Marshal

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I created this function to convert struct to JSON string by ignoring some fields  Hope it will help   func GetJSONString obj interface    ignoreFields    string   string  error        toJson  err    json Marshal obj      if err    nil           return     err            if len ignoreFields     0           return string toJson   nil            toMap    map string interface         json Unmarshal   byte string toJson     amp toMap       for    field    range ignoreFields           delete toMap  field             toJson  err   json Marshal toMap      if err    nil           return     err           return string toJson   nil     Example  https   play golang org p nmq7MFF47Gp

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I didn t have the same problem but similar  Below code solves your problem too  of course if you don t mind performance issue  Before implement that kind of solution to your system I recommend you to redesign your structure if you can  Sending variable structure response is over-engineering  I believe a response structure represents a contract between a request and resource and it should t be depend requests  you can make un-wanted fields null  I do   In some cases we have to implement this design  if you believe you are in that cases here is the play link and code I use   type User2 struct       ID       int     groups  id  json  id omitempty       Username string  groups  username  json  username omitempty       Nickname string  groups  nickname  json  nickname omitempty      type User struct       ID       int     groups  private public  json  id omitempty       Username string  groups  private  json  username omitempty       Nickname string  groups  public  json  nickname omitempty      var       tagName    groups       OmitFields sets fields nil by checking their tag group value and access control tags acTags  func OmitFields obj interface    acTags   string          nilV    reflect Value       sv    reflect ValueOf obj  Elem       st    sv Type       if sv Kind      reflect Struct           for i    0  i  lt  st NumField    i                 fieldVal    sv Field i              if fieldVal CanSet                     tagStr    st Field i  Tag Get tagName                  if len tagStr     0                       continue                                   tagList    strings Split strings Replace tagStr           -1                          fmt Println tagList                     ContainsCommonItem checks whether there is at least one common item in arrays                 if  ContainsCommonItem tagList  acTags                        fieldVal Set reflect Zero fieldVal Type                                                          ContainsCommonItem checks if arrays have at least one equal item func ContainsCommonItem arr1   string  arr2   string  bool       for i    0  i  lt  len arr1   i             for j    0  j  lt  len arr2   j                 if arr1 i     arr2 j                    return true                                   return false   func main         u    User ID  1  Username   very secret   Nickname   hinzir         assume authenticated user doesn t has permission to access private fields     OmitFields  amp u    string  public         bytes       json Marshal  amp u      fmt Println string bytes         u2    User2 ID  1  Username   very secret   Nickname   hinzir         you want to filter fields by field names     OmitFields  amp u2    string  id    nickname         bytes      json Marshal  amp u2      fmt Println string bytes

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You can use the reflect package to select the fields that you want by reflecting on the field tags and selecting the json tag values   Define a method on your SearchResults type that selects the fields you want and returns them as a map string interface    and then marshal that instead of the SearchResults struct itself   Here s an example of how you might define that method   func fieldSet fields    string  map string bool       set    make map string bool  len fields       for    s    range fields           set s    true           return set    func  s  SearchResult  SelectFields fields    string  map string interface         fs    fieldSet fields         rt  rv    reflect TypeOf  s   reflect ValueOf  s      out    make map string interface    rt NumField        for i    0  i  lt  rt NumField    i             field    rt Field i          jsonKey    field Tag Get  json           if fs jsonKey                out jsonKey    rv Field i  Interface                       return out     and here s a runnable solution that shows how you would call this method and marshal your selection   http   play golang org p 1K9xjQRnO8

[json] Removing fields from struct or hiding them in JSON Response

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