Allow encoding data with missing fields.

README.md

@@ -180,7 +180,25 @@ file:close()
   local output = ftcsv.encode(everyUser, {encodeNilAs=0}) -- for setting it to 0
   ```
 
+- `allowMissingKeys`
 
+  If set to a non-`nil` value, this option allows encoding data sets that are entirely missing a field that was specified in `fieldsToKeep`. Otherwise, ftcsv would raise an error.
+
+  ```lua
+  local data = {
+    {a = 1, b = 2, c = 3},
+    {a = 10, b = 20},
+    {a = 100, c = 200},
+  }
+  ftcsv.encode(data, {fieldsToKeep = {"a", "b", "c", "d"}}) --> [throws an error]
+  ftcsv.encode(data, {fieldsToKeep = {"a", "b", "c", "d"}, allowMissingKeys = true})
+  --> [[
+  --> "a","b","c","d"
+  --> "1","2","3","nil"
+  --> "10","20","nil","nil"
+  --> "100","nil","200","nil"
+  --> ]]
+  ```
 
 ## Error Handling
 ftcsv returns a litany of errors when passed a bad csv file or incorrect parameters. You can find a more detailed explanation of the more cryptic errors in [ERRORS.md](ERRORS.md)

ftcsv.lua

@@ -814,7 +814,9 @@ local function initializeGenerator(inputTable, delimiter, options)
     if headers == nil then
         headers = extractHeadersFromTable(inputTable)
     end
-    validateHeaders(headers, inputTable)
+    if options and options.allowMissingKeys == nil then
+        validateHeaders(headers, inputTable)
+    end
 
     local escapedHeaders = escapeHeadersForOutput(headers, delimiter, options)
     local output = initializeOutputWithEscapedHeaders(escapedHeaders, delimiter, options)

spec/csvs/missing_keys.csv

@@ -0,0 +1,4 @@
+a,b,c
+1,2,3
+10,20,
+100,,300

spec/json/missing_keys.json

@@ -0,0 +1,22 @@
+[
+    {
+        "a": "1",
+        "b": "2",
+        "c": "3",
+        "d": "nil"
+    },
+    {
+        "a": "10",
+        "b": "20",
+        "c": "nil",
+        "d": "nil"
+
+    },
+    {
+        "a": "100",
+        "b": "nil",
+        "c": "300",
+        "d": "nil"
+
+    }
+]

spec/parse_encode_spec.lua

@@ -114,3 +114,35 @@ describe("csv encode without quotes", function()
 		end)
 	end
 end)
+
+--[[ This breaks simple_crlf.
+describe("csv encode with missing keys", function()
+	for _, value in ipairs(files) do
+		it("should handle " .. value, function()
+			local jsonFile = loadFile("spec/json/" .. value .. ".json")
+			local jsonDecode = cjson.decode(jsonFile)
+			local reEncoded = ftcsv.parse(ftcsv.encode(
+				jsonDecode, ",", {
+					fieldsToKeep = {"a", "b", "c", "d"},
+					allowMissingKeys = true,
+				}
+			), ",", {loadFromString=true})
+			assert.are.same(jsonDecode, reEncoded)
+		end)
+	end
+end)
+--]]
+
+describe("csv encode with missing keys", function()
+	it("should handle missing_keys", function()
+		local jsonFile = loadFile("spec/json/missing_keys.json")
+		local jsonDecode = cjson.decode(jsonFile)
+		local reEncoded = ftcsv.parse(ftcsv.encode(
+			jsonDecode, ",", {
+				fieldsToKeep = {"a", "b", "c", "d"},
+				allowMissingKeys = true,
+			}
+		), ",", {loadFromString=true})
+		assert.are.same(jsonDecode, reEncoded)
+	end)
+end)