To store in json :

JSON | Article | database_field_column [field_name]

It works very well. 

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Exemple : 

JSON | Article | attribs[field_name] 

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It will Output : attribs{"field_name":"myvalue"}

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For my needs, I need to store attribs with multilevel.

I want to output something like this :

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attribs{"parent_field_name":{"field_name":"myvalue"}}

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I hardly tried to figure it out by myself but .. didn't find the synthax.

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Then, with BeforeStore field, would I be able to save something like this in JSON :

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attribs{"parent_field_name":{"field_name":"myvalue","field_name2":"myvalue2","field_name3":"myvalue3"}}