mongodb/jstests/slice1.js

t = db.slice1;
t.drop();

t.insert({_id:1, a:[0,1,2,3,4,5,-5,-4,-3,-2,-1], b:1, c:1});

// first three
out = t.findOne({}, {a:{$slice:3}});
assert.eq(out.a , [0,1,2], '1');

// last three
out = t.findOne({}, {a:{$slice:-3}});
assert.eq(out.a , [-3, -2, -1], '2');

// skip 2, limit 3
out = t.findOne({}, {a:{$slice:[2, 3]}});
assert.eq(out.a , [2,3,4], '3');

// skip to fifth from last, limit 4
out = t.findOne({}, {a:{$slice:[-5, 4]}});
assert.eq(out.a , [-5, -4, -3, -2], '4');

// skip to fifth from last, limit 10
out = t.findOne({}, {a:{$slice:[-5, 10]}});
assert.eq(out.a , [-5, -4, -3, -2, -1], '5');


// interaction with other fields

out = t.findOne({}, {a:{$slice:3}});
assert.eq(out.a , [0,1,2], 'A 1');
assert.eq(out.b , 1, 'A 2');
assert.eq(out.c , 1, 'A 3');

out = t.findOne({}, {a:{$slice:3}, b:true});
assert.eq(out.a , [0,1,2], 'B 1');
assert.eq(out.b , 1, 'B 2');
assert.eq(out.c , undefined);

out = t.findOne({}, {a:{$slice:3}, b:false});
assert.eq(out.a , [0,1,2]);
assert.eq(out.b , undefined);
assert.eq(out.c , 1);

t.drop()
t.insert({comments: [{id:0, text:'a'},{id:1, text:'b'},{id:2, text:'c'},{id:3, text:'d'}], title:'foo'})


out = t.findOne({}, {comments:{$slice:2}, 'comments.id':true});
assert.eq(out.comments , [{id:0}, {id:1}]);
assert.eq(out.title , undefined);

out = t.findOne({}, {comments:{$slice:2}, 'comments.id':false});
assert.eq(out.comments , [{text: 'a'}, {text: 'b'}]);
assert.eq(out.title , 'foo');

//nested arrays
t.drop();
t.insert({_id:1, a:[[1,1,1], [2,2,2], [3,3,3]], b:1, c:1});

out = t.findOne({}, {a:{$slice:1}});
assert.eq(out.a , [[1,1,1]], 'n 1');

out = t.findOne({}, {a:{$slice:-1}});
assert.eq(out.a , [[3,3,3]], 'n 2');

out = t.findOne({}, {a:{$slice:[0,2]}});
assert.eq(out.a , [[1,1,1],[2,2,2]], 'n 2');