DRV-368: Added support for omitempty field tag

[mscno]

This commit fixes #123 and enables discarding zero valued fields when saving structs that are tagged with omitempty

Test is included, existing functionality with tagging with a name and ignoring with - is maintained

func TestSerializeStructWithOmitEmptyTags(t *testing.T) {
	type user struct {
		Name     string  `fauna:"name,omitempty"`
		LastName string  `fauna:"last_name,omitempty"`
		Age      int     `fauna:"age,omitempty"`
		Siblings int     `fauna:"siblings,omitempty"`
		Height   float64 `fauna:"height,omitempty"`
		Weight   float64 `fauna:"weight,omitempty"`
	}

	assertJSON(t,
		Obj{"data": user{"Jhon", "", 18, 0, 123.123, 0.0}},
		`{"object":{"data":{"object":{"age":18, "height":123.123, "name":"Jhon"}}}}`,
	)
}

[mscno]

tests and fixes for deserialization still needed, will get that one of the next days

[mscno]

I added a WIP of sucessfully decoding omitempty structs.

This however caused another test to fail:

TestRunClientTests/TestEvalCountMeanSumExpression

Probably need someone with more intricate knowledge of the FQL language and serialization/deserialization to finish and fix this

[marrony]

@mscno sorry but what's the purpose of the tag? I could not identify by the tests. Is it to omit the field from json serialization?

[mscno]

hi @marrony, yes the purpose is to avoid serialization of the json structs to allow for partial update of a document. Currently when doing partial updates using a struct, the zero values will overwrite all the existing values in the document. This requires users that want to do partial updates to a document, to do the update manually using FQL object constructors.

I gave a more thorough description in the issue: #123

Other databases (Google Cloud Datastore/Firestore) and Pg-Go (Postgres) drivers has support for ignoring zero values, which enables you do to a partial update on a document/row without sending all existing values over the wire.

[marrony]

can you enhance this struct to make it clear that the empty values will not be serialized (btw, it was really hard to see that weight_pointer field was meant to omitted ), you don't need to a "real" user struct, I'm more interested to see the all the combinations of data types you can omit, something along in the lines

	type omitStruct struct {
		String string `fauna:str,omitempty`,
		Int *int `fauna:int,omitempty`
		....
	}

	assertJSON(t,
 		Obj{"data": omitStruct{nil, nil, ....}},
 		`{"object":{"data":{"object":{}}}}`,
 	)

	assertJSON(t,
 		Obj{"data": omitStruct{"String", nil, ....}},
 		`{"object":{"data":{"object":{"str":"String"}}}}`,
 	)

	x := 10
	assertJSON(t,
 		Obj{"data": omitStruct{nil, &x, ....}},
 		`{"object":{"data":{"object":{"int":10}}}}`,
 	)

 	//more assertions

[mscno]

certainly, ill update the tests to make it more clear how it handles different empty values.

[parkhomenko]

hi @mscno,
sorry for bothering you, but do you have plans on wrapping up this PR?

[vadimLF]

This was done with #134. This PR can be closed.