I have a large file (>50Mb) which contains a JSON hash. Something like:
{
"obj1": {
"key1": "val1",
"key2": "val2"
},
"obj2": {
"key1": "val1",
"key2": "val2"
}
...
}
Rather than parsing the entire file and taking say the first ten elements, I'd like to parse each item in the hash. I actually don't care about the key, i.e. obj1
.
If I convert the above to this:
{
"key1": "val1",
"key2": "val2"
}
"obj2": {
"key1": "val1",
"key2": "val2"
}
I can easily achieve what I want using Yajl streaming:
io = File.open(path_to_file)
count = 10
Yajl::Parser.parse(io) do |obj|
puts "Parsed: #{obj}"
count -= 1
break if count == 0
end
io.close
Is there a way to do this without having to alter the file? Some sort of callback in Yajl maybe?
I ended up solving this using JSON::Stream which has callbacks for start_document
, start_object
etc.
I gave my 'parser' a to_enum
method which emits all the 'Resource' objects as they're parsed. Note that ResourcesCollectionNode
is never really used unless you completely parse the JSON stream, and the ResourceNode
is a subclass of ObjectNode
for naming purposes only, though I might just get rid of it:
class Parser
METHODS = %w[start_document end_document start_object end_object start_array end_array key value]
attr_reader :result
def initialize(io, chunk_size = 1024)
@io = io
@chunk_size = chunk_size
@parser = JSON::Stream::Parser.new
# register callback methods
METHODS.each do |name|
@parser.send(name, &method(name))
end
end
def to_enum
Enumerator.new do |yielder|
@yielder = yielder
begin
while [email protected]?
# puts "READING CHUNK"
chunk = @io.read(@chunk_size)
@parser << chunk
end
ensure
@yielder = nil
end
end
end
def start_document
@stack = []
@result = nil
end
def end_document
# @result = @stack.pop.obj
end
def start_object
if @stack.size == 0
@stack.push(ResourceCollectionNode.new)
elsif @stack.size == 1
@stack.push(ResourceNode.new)
else
@stack.push(ObjectNode.new)
end
end
def end_object
if @stack.size == 2
node = @stack.pop
#puts "Stack depth: #{@stack.size}. Node: #{node.class}"
@stack[-1] << node.obj
# puts "Parsed complete resource: #{node.obj}"
@yielder << node.obj
elsif @stack.size == 1
# puts "Parsed all resources"
@result = @stack.pop.obj
else
node = @stack.pop
# puts "Stack depth: #{@stack.size}. Node: #{node.class}"
@stack[-1] << node.obj
end
end
def end_array
node = @stack.pop
@stack[-1] << node.obj
end
def start_array
@stack.push(ArrayNode.new)
end
def key(key)
# puts "Stack depth: #{@stack.size} KEY: #{key}"
@stack[-1] << key
end
def value(value)
node = @stack[-1]
node << value
end
class ObjectNode
attr_reader :obj
def initialize
@obj, @key = {}, nil
end
def <<(node)
if @key
@obj[@key] = node
@key = nil
else
@key = node
end
self
end
end
class ResourceNode < ObjectNode
end
# Node that contains all the resources - a Hash keyed by url
class ResourceCollectionNode < ObjectNode
def <<(node)
if @key
@obj[@key] = node
# puts "Completed Resource: #{@key} => #{node}"
@key = nil
else
@key = node
end
self
end
end
class ArrayNode
attr_reader :obj
def initialize
@obj = []
end
def <<(node)
@obj << node
self
end
end
end
and an example in use:
def json
<<-EOJ
{
"1": {
"url": "url_1",
"title": "title_1",
"http_req": {
"status": 200,
"time": 10
}
},
"2": {
"url": "url_2",
"title": "title_2",
"http_req": {
"status": 404,
"time": -1
}
},
"3": {
"url": "url_1",
"title": "title_1",
"http_req": {
"status": 200,
"time": 10
}
},
"4": {
"url": "url_2",
"title": "title_2",
"http_req": {
"status": 404,
"time": -1
}
},
"5": {
"url": "url_1",
"title": "title_1",
"http_req": {
"status": 200,
"time": 10
}
},
"6": {
"url": "url_2",
"title": "title_2",
"http_req": {
"status": 404,
"time": -1
}
}
}
EOJ
end
io = StringIO.new(json)
resource_parser = ResourceParser.new(io, 100)
count = 0
resource_parser.to_enum.each do |resource|
count += 1
puts "READ: #{count}"
pp resource
break
end
io.close
Output:
READ: 1
{"url"=>"url_1", "title"=>"title_1", "http_req"=>{"status"=>200, "time"=>10}}
I faced the same problem and created the gem json-streamer that will save you the need to create your own callbacks.
The usage in your case would be (v 0.4.0):
io = File.open(path_to_file)
streamer = Json::Streamer::JsonStreamer.new(io)
streamer.get(nesting_level:1).each do |object|
p oject
end
io.close
Applying it on your example it will yield the objects without the 'obj' keys:
{
"key1": "val1",
"key2": "val2"
}
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