yq/pkg/yqlib/doc/Multiply (Merge).md

Like the multiple operator in jq, depending on the operands, this multiply operator will do different things. Currently numbers, arrays and objects are supported.

## Objects and arrays - merging
Objects are merged deeply matching on matching keys. By default, array values override and are not deeply merged.

Note that when merging objects, this operator returns the merged object (not the parent). This will be clearer in the examples below.

### Merge Flags
You can control how objects are merged by using one or more of the following flags. Multiple flags can be used together, e.g. `.a *+? .b`.  See examples below

- `+` to append arrays
- `?` to only merge existing fields
- `d` to deeply merge arrays

### Merging files
Note the use of `eval-all` to ensure all documents are loaded into memory.

```bash
yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' file1.yaml file2.yaml
```

## Multiply integers
Running
```bash
yq eval --null-input '3 * 4'
```
will output
```yaml
12
```

## Merge objects together, returning merged result only
Given a sample.yml file of:
```yaml
a:
  field: me
  fieldA: cat
b:
  field:
    g: wizz
  fieldB: dog
```
then
```bash
yq eval '.a * .b' sample.yml
```
will output
```yaml
field:
  g: wizz
fieldA: cat
fieldB: dog
```

## Merge objects together, returning parent object
Given a sample.yml file of:
```yaml
a:
  field: me
  fieldA: cat
b:
  field:
    g: wizz
  fieldB: dog
```
then
```bash
yq eval '. * {"a":.b}' sample.yml
```
will output
```yaml
a:
  field:
    g: wizz
  fieldA: cat
  fieldB: dog
b:
  field:
    g: wizz
  fieldB: dog
```

## Merge keeps style of LHS
Given a sample.yml file of:
```yaml
a: {things: great}
b:
  also: "me"
```
then
```bash
yq eval '. * {"a":.b}' sample.yml
```
will output
```yaml
a: {things: great, also: "me"}
b:
  also: "me"
```

## Merge arrays
Given a sample.yml file of:
```yaml
a:
  - 1
  - 2
  - 3
b:
  - 3
  - 4
  - 5
```
then
```bash
yq eval '. * {"a":.b}' sample.yml
```
will output
```yaml
a:
  - 3
  - 4
  - 5
b:
  - 3
  - 4
  - 5
```

## Merge, only existing fields
Given a sample.yml file of:
```yaml
a:
  thing: one
  cat: frog
b:
  missing: two
  thing: two
```
then
```bash
yq eval '.a *? .b' sample.yml
```
will output
```yaml
thing: two
cat: frog
```

## Merge, appending arrays
Given a sample.yml file of:
```yaml
a:
  array:
    - 1
    - 2
    - animal: dog
  value: coconut
b:
  array:
    - 3
    - 4
    - animal: cat
  value: banana
```
then
```bash
yq eval '.a *+ .b' sample.yml
```
will output
```yaml
array:
  - 1
  - 2
  - animal: dog
  - 3
  - 4
  - animal: cat
value: banana
```

## Merge, only existing fields, appending arrays
Given a sample.yml file of:
```yaml
a:
  thing:
    - 1
    - 2
b:
  thing:
    - 3
    - 4
  another:
    - 1
```
then
```bash
yq eval '.a *?+ .b' sample.yml
```
will output
```yaml
thing:
  - 1
  - 2
  - 3
  - 4
```

## Merge, deeply merging arrays
Merging arrays deeply means arrays are merge like objects, with indexes as their key. In this case, we merge the first item in the array, and do nothing with the second.

Given a sample.yml file of:
```yaml
a:
  - name: fred
    age: 12
  - name: bob
    age: 32
b:
  - name: fred
    age: 34
```
then
```bash
yq eval '.a *d .b' sample.yml
```
will output
```yaml
- name: fred
  age: 34
- name: bob
  age: 32
```

## Merge arrays of objects together, matching on a key
It's a complex command, the trickyness comes from needing to have the right context in the expressions.
First we save the second array into a variable '$two' which lets us reference it later.
We then need to update the first array. We will use the relative update (|=) because we need to update relative to the current element of the array in the LHS in the RHS expression. 
We set the current element of the first array as $cur. Now we multiply (merge) $cur with the matching entry in $two, by passing $two through a select filter.


Given a sample.yml file of:
```yaml
- a: apple
  b: appleB
- a: kiwi
  b: kiwiB
- a: banana
  b: bananaB
```
And another sample another.yml file of:
```yaml
- a: banana
  c: bananaC
- a: apple
  b: appleB2
- a: dingo
  c: dingoC
```
then
```bash
yq eval-all '(select(fi==1) | .[]) as $two | select(fi==0) | .[] |= (. as $cur |  $cur * ($two | select(.a == $cur.a)))' sample.yml another.yml
```
will output
```yaml
- a: apple
  b: appleB2
- a: kiwi
  b: kiwiB
- a: banana
  b: bananaB
  c: bananaC
```

## Merge to prefix an element
Given a sample.yml file of:
```yaml
a: cat
b: dog
```
then
```bash
yq eval '. * {"a": {"c": .a}}' sample.yml
```
will output
```yaml
a:
  c: cat
b: dog
```

## Merge with simple aliases
Given a sample.yml file of:
```yaml
a: &cat
  c: frog
b:
  f: *cat
c:
  g: thongs
```
then
```bash
yq eval '.c * .b' sample.yml
```
will output
```yaml
g: thongs
f: *cat
```

## Merge copies anchor names
Given a sample.yml file of:
```yaml
a:
  c: &cat frog
b:
  f: *cat
c:
  g: thongs
```
then
```bash
yq eval '.c * .a' sample.yml
```
will output
```yaml
g: thongs
c: &cat frog
```

## Merge with merge anchors
Given a sample.yml file of:
```yaml
foo: &foo
  a: foo_a
  thing: foo_thing
  c: foo_c
bar: &bar
  b: bar_b
  thing: bar_thing
  c: bar_c
foobarList:
  b: foobarList_b
  !!merge <<:
    - *foo
    - *bar
  c: foobarList_c
foobar:
  c: foobar_c
  !!merge <<: *foo
  thing: foobar_thing
```
then
```bash
yq eval '.foobar * .foobarList' sample.yml
```
will output
```yaml
c: foobarList_c
<<:
  - *foo
  - *bar
thing: foobar_thing
b: foobarList_b
```