[SPARK-19849][SQL] Support ArrayType in to_json to produce JSON array

## What changes were proposed in this pull request?

This PR proposes to support an array of struct type in `to_json` as below:

```scala
import org.apache.spark.sql.functions._

val df = Seq(Tuple1(Tuple1(1) :: Nil)).toDF("a")
df.select(to_json($"a").as("json")).show()
```

```
+----------+
|      json|
+----------+
|[{"_1":1}]|
+----------+
```

Currently, it throws an exception as below (a newline manually inserted for readability):

```
org.apache.spark.sql.AnalysisException: cannot resolve 'structtojson(`array`)' due to data type
mismatch: structtojson requires that the expression is a struct expression.;;
```

This allows the roundtrip with `from_json` as below:

```scala
import org.apache.spark.sql.functions._
import org.apache.spark.sql.types._

val schema = ArrayType(StructType(StructField("a", IntegerType) :: Nil))
val df = Seq("""[{"a":1}, {"a":2}]""").toDF("json").select(from_json($"json", schema).as("array"))
df.show()

// Read back.
df.select(to_json($"array").as("json")).show()
```

```
+----------+
|     array|
+----------+
|[[1], [2]]|
+----------+

+-----------------+
|             json|
+-----------------+
|[{"a":1},{"a":2}]|
+-----------------+
```

Also, this PR proposes to rename from `StructToJson` to `StructsToJson ` and `JsonToStruct` to `JsonToStructs`.

## How was this patch tested?

Unit tests in `JsonFunctionsSuite` and `JsonExpressionsSuite` for Scala, doctest for Python and test in `test_sparkSQL.R` for R.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17192 from HyukjinKwon/SPARK-19849.

2 files changed, 16 insertions, 6 deletions

diff --git a/R/pkg/R/functions.R b/R/pkg/R/functions.R
index 9867f2d5b7..2cff3ac08c 100644
--- a/R/pkg/R/functions.R
+++ b/R/pkg/R/functions.R
@@ -1795,10 +1795,10 @@ setMethod("to_date",
 
 #' to_json
 #'
-#' Converts a column containing a \code{structType} into a Column of JSON string.
-#' Resolving the Column can fail if an unsupported type is encountered.
+#' Converts a column containing a \code{structType} or array of \code{structType} into a Column
+#' of JSON string. Resolving the Column can fail if an unsupported type is encountered.
 #'
-#' @param x Column containing the struct
+#' @param x Column containing the struct or array of the structs
 #' @param ... additional named properties to control how it is converted, accepts the same options
 #'            as the JSON data source.
 #'
@@ -1809,8 +1809,13 @@ setMethod("to_date",
 #' @export
 #' @examples
 #' \dontrun{
-#' to_json(df$t, dateFormat = 'dd/MM/yyyy')
-#' select(df, to_json(df$t))
+#' # Converts a struct into a JSON object
+#' df <- sql("SELECT named_struct('date', cast('2000-01-01' as date)) as d")
+#' select(df, to_json(df$d, dateFormat = 'dd/MM/yyyy'))
+#'
+#' # Converts an array of structs into a JSON array
+#' df <- sql("SELECT array(named_struct('name', 'Bob'), named_struct('name', 'Alice')) as people")
+#' select(df, to_json(df$people))
 #'}
 #' @note to_json since 2.2.0
 setMethod("to_json", signature(x = "Column"),
@@ -2433,7 +2438,8 @@ setMethod("date_format", signature(y = "Column", x = "character"),
 #' from_json
 #'
 #' Parses a column containing a JSON string into a Column of \code{structType} with the specified
-#' \code{schema}. If the string is unparseable, the Column will contains the value NA.
+#' \code{schema} or array of \code{structType} if \code{asJsonArray} is set to \code{TRUE}.
+#' If the string is unparseable, the Column will contains the value NA.
 #'
 #' @param x Column containing the JSON string.
 #' @param schema a structType object to use as the schema to use when parsing the JSON string.
diff --git a/R/pkg/inst/tests/testthat/test_sparkSQL.R b/R/pkg/inst/tests/testthat/test_sparkSQL.R
index 32856b399c..9c38e0d866 100644
--- a/R/pkg/inst/tests/testthat/test_sparkSQL.R
+++ b/R/pkg/inst/tests/testthat/test_sparkSQL.R
@@ -1340,6 +1340,10 @@ test_that("column functions", {
   expect_equal(collect(select(df, bround(df$x, 0)))[[1]][2], 4)
 
   # Test to_json(), from_json()
+  df <- sql("SELECT array(named_struct('name', 'Bob'), named_struct('name', 'Alice')) as people")
+  j <- collect(select(df, alias(to_json(df$people), "json")))
+  expect_equal(j[order(j$json), ][1], "[{\"name\":\"Bob\"},{\"name\":\"Alice\"}]")
+
   df <- read.json(mapTypeJsonPath)
   j <- collect(select(df, alias(to_json(df$info), "json")))
   expect_equal(j[order(j$json), ][1], "{\"age\":16,\"height\":176.5}")