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https://issues.apache.org/jira/browse/AVRO-2597?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17256217#comment-17256217
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Joel Turkel commented on AVRO-2597:
-----------------------------------
I hit this issue trying use the Avro C++ library to speed up Avro
serialization/deserialization for the Ruby
[avromatic|https://github.com/salsify/avromatic] library. Avromatic has special
handling of "optional" fields (i.e. unions that include a null member type) so
for a given union GenericDatum I need to figure out the number of member types
in the union schema and whether the first member schema is a null type.
Exposing the underlying union schema would make this easier and more efficient
than the workarounds.
I suspect exposing a GenericDatum#schema() method might create confusion since
GenericDatum#type() would return a different result than
GenericDatum#schema()->type(). Perhaps we could add a
GenericDatum#unionSchema() method that would return the underlying union's
schema? It's a little awkward since it would raise an error if the GenericDatum
doesn't correspond to a union but that's consistent with the
GenericDatum#unionBranch() and GenericDatum#selectBranch() methods.
> GenricUnion not accessible
> --------------------------
>
> Key: AVRO-2597
> URL: https://issues.apache.org/jira/browse/AVRO-2597
> Project: Apache Avro
> Issue Type: Bug
> Components: c++
> Affects Versions: 1.9.0, 1.9.1
> Reporter: Marcel Pfütze
> Priority: Major
>
> In version 1.9.0 GenricDatum.hh was expanded by functionality that was
> formerly just in the GenricUnion class.
> The following was added:
> * isUnion()
> * selectBranch()
> * unionBranch()
> Additionally the methods to access type and value were overwritten to have a
> special if case for the union data type:
> {code:c++}
> inline Type GenericDatum::type() const {
> return (type_ == AVRO_UNION) ?
> #if __cplusplus >= 201703L
> std::any_cast<GenericUnion>(&value_)->datum().type() :
> #else
> boost::any_cast<GenericUnion>(&value_)->datum().type() :
> #endif
> type_;
> }
> template<typename T> T& GenericDatum::value() {
> return (type_ == AVRO_UNION) ?
> #if __cplusplus >= 201703L
> std::any_cast<GenericUnion>(&value_)->datum().value<T>() :
> *std::any_cast<T>(&value_);
> #else
> boost::any_cast<GenericUnion>(&value_)->datum().value<T>() :
> *boost::any_cast<T>(&value_);
> #endif
> }
> {code}
> The result of this is, that it's impossible to access the GenericUnion from a
> GenricDatum as the code always points to the branch of the union.
> Currently the only way to get the number of branches this union has, is by
> calling selectbranch() in a for-loop until the underlying leafAt() throws an
> exception.
> In avro 1.8.2 the following code could be used to select a unionbranch of a
> specific type.
> {code:c++}
> inline avro::GenericDatum & select_union_branch(avro::GenericDatum & datum,
> const avro::Type type)
> {
> auto & un = datum.value<avro::GenericUnion>();
> auto & schema = un.schema();
> auto leaves = schema->leaves();
> for(std::size_t i=0; i < leaves; i++) {
> if(schema->leafAt(i)->type() == type) {
> un.selectBranch(i);
> return un.datum();
> }
> }
> throw std::runtime_error(
> "select_union_branch: can't find branch with type=" +
> std::to_string(type));
> }
> {code}
> My proposals:
> # Revert changes to make GenericDatum reflect methods of GenericUnion.
> ## A Union is a normal data type of Avro like Records or Lists. What is the
> point of hiding the type?
> ## The getters don't just return their value but do an extra check or even
> cast before returning. This is counterintuitive.
> # Add the schema() method to the GenericDatum class. At least the full
> functionality of the GenricUnion should be accessible if you can't access the
> GenericUnion itself.
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