select few elements of an inner array using jq - json

Question

I want create a more simple json with the same original structure but with one a small sample.
As example, If I have this json:
{
"field1": [
{
"a": "F1A1",
"b": "F1B1"
},
{
"a": "F1A2",
"b": "F1B2"
},
{
"a": "F1A3",
"b": "F1B3"
},
{
"a": "F1A4",
"b": "F1B4"
}
],
"field2": [
{
"a": "F2A1",
"b": "F2B1"
},
{
"a": "F2A2",
"b": "F2B2"
}
],
"field3": [
{
"a": "F3A1",
"b": "F3B1"
},
{
"a": "F3A2",
"b": "F3B2"
}
]
}
I want to get the first array element from the first field. So I was expecting this:
{
"field1": [
{
"a": "F1A1",
"b": "F1B1"
}
],
}
I executed jq "select(.field1[0])" tmp.json but it returns the original json.
Bonus:
As bonus, how to do the same but extracting let's say field1 and elements in the array with a=="F1A1" and a=="F1A4", so will expect?:
{
"field1": [
{
"a": "F1A1",
"b": "F1B1"
},
{
"a": "F1A4",
"b": "F1B4"
}
]
}

Answer 1

reduce the oouter object to your field using {field1}, then map this field to an array containing only the first item:
jq '{field1} | map_values([first])'
{
"field1": [
{
"a": "F1A1",
"b": "F1B1"
}
]
}
To filter for certain items use select:
jq '{field1} | map_values(map(select(.a == "F1A1" or .a == "F1A4")))'
{
"field1": [
{
"a": "F1A1",
"b": "F1B1"
},
{
"a": "F1A4",
"b": "F1B4"
}
]
}
As you can see, select does something different. It passes on its input if the argument evaluates to true. Therefore its output is either all or nothing, never just a filtered part. (Of course, you can use select to achieve specific filtering, as shown above.)